ARM: dts: fixed gpio key node for exynos4412-trats2

This patch fixed gpio key device node.
First, fix incorrect gpio property.
And then, add ok-key node where locate bottom center.
I have tested on exynos4412-trats2 board.

Signed-off-by: Beomho Seo <beomho.seo@samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>