pinctrl: Add pinctrl driver support for Exynos7420 SoC

Add pinctrl driver support for Samsung's Exynos7420 SoC. The changes
have been split into Exynos7420 specific and common Exynos specific
portions so that this implementation is reusable on other Exynos
SoCs as well.

The Exynos pinctrl driver supports only device tree based pin
configuration. The bindings used are similar to the ones used in the
linux kernel.

Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Signed-off-by: Thomas Abraham <thomas.ab@samsung.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Minkyu Kang <mk7.kang@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>