ARM: dts: add 'hdmi' clock to mixer nodes for exynos5250 and exynos5420

Mixed block needs to control hdmi clock to properly perform power on/off
operation, so add 'hdmi' clock also to mixer nodes.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>