pinctrl: rockchip: save and restore gpio6_c6 pinmux in suspend/resume

Save and restore the gpio6_c6 pinmux setting, since Maskrom of RK3288
would modify it to sdmmc0_det, so it need to be restored to the correct
setting after resume from Maskrom.

Signed-off-by: Chris Zhong <zyw@rock-chips.com>
Tested-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>