ARM: dts: add support for GPIO buttons for exynos4412-odroid

This patch adds support for simple GPIO-based button availabled on
Exynos4 based Odroid boards. All supported boards have POWER button,
which has been defined in exynos4412-odroid-common.dtsi. X/X2 boards
also have additional user-configurable button which has been mapped to
KEY_HOME. All defined keys have been marked as possible wakeup source.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>