ARM: dts: Remove g2d_pd node for exynos5420

G2D power domain also controls the CMU block of G2D. Since
clock registers can be accessed anytime for viewing
clk_summary, it can cause a system crash if g2d power domain
is disabled.

Signed-off-by: Arun Kumar K <arun.kk@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>