dt-bindings: at24: new optional property - wp-gpios

AT24 EEPROMs have a write-protect pin, which - when pulled high -
inhibits writes to the upper quadrant of memory (although it has been
observed that on some chips it disables writing to the entire memory
range).

On some boards, this pin is connected to a GPIO and pulled high by
default, which forces the user to manually change its state before
writing. On linux this means that we either need to hog the line all
the time, or set the GPIO value before writing from outside of the
at24 driver.

Add a new optional property to the device tree binding document, which
allows to specify the GPIO line to which the write-protect pin is
connected.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>