of/documentation: update with clock information for exynos hdmi subsystem

Adding information about clocks to the binding documentation
for exynos mixer and hdmi.

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>