drivers/gpio: gpio-nomadik: Provide documentation for Device Tree bindings

Add required documentation for specific gpio-nomadik DT bindings.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>