ARM: dts: change to correct compatible string for exynos5420 hdmi

Replace compatible string for HDMI node in Exynos5420. Since
latest restructring in Drm hdmi driver, it is agreed to use
a seperate compatible string for Exynos5420 HDMI IP siince it
uses APB mapped Phy.

Signed-off-by: Rahul Sharma <rahul.sharma@samsung.com>
Acked-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>