gpio: mmio: add MyBook Live GPIO support

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

The GPIOs will be supported by the generic driver
for memory-mapped GPIO controllers.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Acked-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>