iio: ad5755: Add DT binding documentation

Signed-off-by: Sean Nyekjaer <sean.nyekjaer@prevas.dk>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

diff --git a/Documentation/devicetree/bindings/iio/dac/ad5755.txt b/Documentation/devicetree/bindings/iio/dac/ad5755.txt
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+* Analog Device AD5755 IIO Multi-Channel DAC Linux Driver
+
+Required properties:
+ - compatible: Has to contain one of the following:
+       adi,ad5755
+       adi,ad5755-1
+       adi,ad5757
+       adi,ad5735
+       adi,ad5737
+
+ - reg: spi chip select number for the device
+ - spi-cpha or spi-cpol: is the only modes that is supported
+
+Recommended properties:
+ - spi-max-frequency: Definition as per
+               Documentation/devicetree/bindings/spi/spi-bus.txt
+
+Optional properties:
+See include/dt-bindings/iio/ad5755.h
+ - adi,ext-dc-dc-compenstation-resistor: boolean set if the hardware have an
+                                        external resistor and thereby bypasses
+                                        the internal compensation resistor.
+ - adi,dc-dc-phase:
+       Valid values for DC DC Phase control is:
+       0: All dc-to-dc converters clock on the same edge.
+       1: Channel A and Channel B clock on the same edge,
+          Channel C and Channel D clock on opposite edges.
+       2: Channel A and Channel C clock on the same edge,
+          Channel B and Channel D clock on opposite edges.
+       3: Channel A, Channel B, Channel C, and Channel D
+          clock 90 degrees out of phase from each other.
+ - adi,dc-dc-freq-hz:
+       Valid values for DC DC frequency is [Hz]:
+       250000
+       410000
+       650000
+ - adi,dc-dc-max-microvolt:
+       Valid values for the maximum allowed Vboost voltage supplied by
+       the dc-to-dc converter is:
+       23000000
+       24500000
+       27000000
+       29500000
+
+Optional for every channel:
+ - adi,mode:
+       Valid values for DAC modes is:
+       0: 0 V to 5 V voltage range.
+       1: 0 V to 10 V voltage range.
+       2: Plus minus 5 V voltage range.
+       3: Plus minus 10 V voltage range.
+       4: 4 mA to 20 mA current range.
+       5: 0 mA to 20 mA current range.
+       6: 0 mA to 24 mA current range.
+ - adi,ext-current-sense-resistor: boolean set if the hardware a external
+                                  current sense resistor.
+ - adi,enable-voltage-overrange: boolean enable voltage overrange
+ - adi,slew: Array of slewrate settings should contain 3 fields:
+       1: Should be either 0 or 1 in order to enable or disable slewrate.
+       2: Slew rate settings:
+               Valid values for the slew rate update frequency:
+               64000
+               32000
+               16000
+               8000
+               4000
+               2000
+               1000
+               500
+               250
+               125
+               64
+               32
+               16
+               8
+               4
+               0
+       3: Slew step size:
+               Valid values for the step size LSBs:
+               1
+               2
+               4
+               16
+               32
+               64
+               128
+               256
+
+Example:
+dac@0 {
+       #address-cells = <1>;
+       #size-cells = <0>;
+       compatible = "adi,ad5755";
+       reg = <0>;
+       spi-max-frequency = <1000000>;
+       spi-cpha;
+       adi,dc-dc-phase = <0>;
+       adi,dc-dc-freq-hz = <410000>;
+       adi,dc-dc-max-microvolt = <23000000>;
+       channel@0 {
+               reg = <0>;
+               adi,mode = <4>;
+               adi,ext-current-sense-resistor;
+               adi,slew = <0 64000 1>;
+       };
+       channel@1 {
+               reg = <1>;
+               adi,mode = <4>;
+               adi,ext-current-sense-resistor;
+               adi,slew = <0 64000 1>;
+       };
+       channel@2 {
+               reg = <2>;
+               adi,mode = <4>;
+               adi,ext-current-sense-resistor;
+               adi,slew = <0 64000 1>;
+       };
+       channel@3 {
+               reg = <3>;
+               adi,mode = <4>;
+               adi,ext-current-sense-resistor;
+               adi,slew = <0 64000 1>;
+       };
+};