clk: qcom: Support display RCG clocks

Add support for the DSI/EDP/HDMI RCG clocks. With the proper
display driver in place this should allow us to support display
clocks on msm8974 based devices.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>