GPIO: gpio-pxa: fix devicetree functions

Provide an of_xlate function for the PXA GPIO chips and make it work for
devicetree environments.

Successfully tested on a PXA3xx board.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>