gpio: dt-bindings: add brcm,bcm6345-gpio bindings

This patch adds the device tree bindings for the Broadcom's BCM6345
memory-mapped GPIO controllers.

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

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>