ARM: dts: Add missing irq pinctrl for max77686 on smdk5250

This patch adds pinctrl configuration for using configuring gpx3-2 as an
external interrupt from max77686. Though max77686 RTC is enabled and gets
probed by default, it doesnt work as its unable to get interrupt.

This patch makes max77686 RTC work and also configures it as wakeup source.

Signed-off-by: Yadwinder Singh Brar <yadi.brar@samsung.com>
[pankaj.dubey: resubmitted after rebasing to latest kgene tree]
Signed-off-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Tested-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Acked-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>