Documentation: devicetree: Add DT bindings to eeprom_93xx46 driver.

This commit documents bindings to be added to the eeprom_93xx46 driver
which will allow:

  - Device word size and read-only attributes to be specified.
  - A device-specific compatible string for use with Atmel AT93C46D
    EEPROMs.
  - Specifying a GPIO line to function as a 'select' or 'enable' signal
    prior to accessing the EEPROM.

Signed-off-by: Cory Tusar <cory.tusar@pid1solutions.com>
Acked-by: Rob Herring <robh@kernel.org>
Tested-by: Chris Healy <chris.healy@zii.aero>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>