dt-bindings: at25: add reference for the wp-gpios property

As the at25 uses the NVMEM subsystem, and the property is now being
handled, adding reference for it in the device tree binding document,
which allows to specify the GPIO line to which the write-protect pin
is connected.

Signed-off-by: Khouloud Touil <ktouil@baylibre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>