gpio: gpio-rcar: Support S2RAM

This patch adds an implementation that saves and restores the state of
GPIO configuration on suspend and resume.

Signed-off-by: Hien Dang <hien.dang.eb@renesas.com>
Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
[Modify structure of the bank info to simplify a saving registers]
[Remove DEV_PM_OPS macro]
Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>
Tested-by: Nguyen Viet Dung <dung.nguyen.aj@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>