gpio: gpio-xilinx: Add support for suspend and resume

Add support for suspend and resume, pm runtime suspend and resume.
Added free and request calls.

Signed-off-by: Srinivas Neeli <srinivas.neeli@xilinx.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>