gpio: dt-bindings: add wd,mbl-gpio bindings

This patch adds the device tree bindings for the Western Digital's
MyBook Live memory-mapped GPIO controllers.

The gpios will be supported by gpio-mmio code of the
GPIO generic library.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>