ARM: dts: add pinctrl for EC irq of exynos5250-snow

Added pinctrl node for embedded controller (EC) IRQ
on Snow board.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>