drivers/gpio: gpio-nomadik: Device Tree bindings

This creates Device Tree bindings for the Nomadik GPIO driver.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
[Split off from the patch actually implementing the bindings]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>