ARM: ux500: Provide a AB8500 GPIO Device Tree node

Here we're adding a node for the AB8500 GPIO device. This will allow
other DT:ed components to obtain GPIOs for use within their drivers.

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