pinctrl: mvebu: add suspend/resume support to Armada XP pinctrl driver

This commit adds suspend/resume support to the Armada XP pinctrl
driver, by simply saving and restoring the MPP registers.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>