misc/at25, dt: Improve at25 SPI eeprom device tree bindings.

misc/at25, dt: Improve at25 SPI eeprom device tree bindings.

Commit 002176db (misc: at25: Parse dt settings) added device tree
bindings the differ significantly in style from the I2C EEPROM
bindings and don't seem well vetted.  Here I deprecate (but still
support) the "at25,*" properties, and add what I hope is a better
alternative.  These new bindings also happen to be deployed in the
field and were previously submitted for consideration here:

https://lists.ozlabs.org/pipermail/devicetree-discuss/2012-May/015556.html

The advantages of the new bindings are that they are similar to the
I2C EEPROMs and they don't conflate read-only and the address width
modes in a binary encoded blob.

Signed-off-by: David Daney <david.daney@cavium.com>
Cc: Alexandre Pereira da Silva <aletes.xgr@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Michael Hennerich <michael.hennerich@analog.com>
Cc: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>