Add the display, video & graphics clock controller nodes supported on
SC7180.

NOTE: the dispcc needs input clocks from various PHYs that aren't in
the device tree yet.  For now we'll leave these stubbed out with <0>,
which is apparently the magic way to do this.  These clocks aren't
really "optional" and this stubbing out method is apparently the best
way to handle it.
Signed-off-by: Taniya Das <tdas@codeaurora.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20200203103049.v4.15.I1a4b93fb005791e29a9dcf288fc8bd459a555a59@changeidSigned-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>