ARM: dts: Use GPIO constants for flags cells in exynos4120 boards

The board DTS are using numeric values instead of the defined GPIO
constanst to express polarity, use them to make the DTS more clear.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>