clk/exynos5420: add hdmi mux to change parents in hdmi driver

hdmi driver needs to change the parent of hdmi clock
to pixel clock or hdmiphy clock, based on the stability
of hdmiphy. This patch is exposing the mux for changing
the parent.

Signed-off-by: Rahul Sharma <rahul.sharma@samsung.com>
Acked-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>