pinctrl: exynos: Handle suspend/resume of GPIO EINT registers

Some GPIO EINT control registers needs to be preserved across
suspend/resume cycle. This patch extends the driver to take care of
this.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>