ARM: dts: enable hdmi subsystem for exynos5420 smdk board

Add pinctrl node for hdmi hpd gpio pin to exynos5420 smdk board
file. hpd Gpio property is added to the hdmi node. This patch also
adds hdmi ddc node.

Both hdmi device and ddc i2c channel are enabled in
exynos5420-smdk5420.dts

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