ARM: dts: AM43XX: Remove min and max voltage values for dcdc3

ARM: dts: AM43XX: Remove min and max voltage values for dcdc3

dcdc3 supplies to DDR on AM43x series. When we set both
min and max values to the same value. The regulator framework
sets that particular voltage. This is bad as we are changing
the ddr voltage when executing from ddr. Hence remove the min and
max values. The ddr supply voltage shall be set from bootloader
when not executing from ddr and not while executing from kernel.

The previous discussion can be found here:

http://www.spinics.net/lists/devicetree/msg56399.html

Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>