dt-bindings: gpio: Document GPIOs via Moxtet bus

This patch adds documentation of the device tree bindings for GPIOs
on the devices connected via Moxtet bus.

Link: https://lore.kernel.org/r/20190812161118.21476-6-marek.behun@nic.cz


Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>