hdmi driver needs to change the parent of hdmi clock
frequently between pixel clock and hdmiphy clock. hdmiphy is
not stable after power on and for a short interval while changing
the phy configuration. For this duration pixel clock is used to
clock hdmi.

This patch is exposing the mux for changing parent.
Signed-off-by: Rahul Sharma <rahul.sharma@samsung.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>