clk: qcom: clock-gcc-8936: add display clock support for MSM8936

Model the DSI clocks in GCC as a separate device. The byte and pixel clocks
exposed by the DSI PLL device is needed by the GCC DSI clocks. Finally, the
DSI controller can get the clocks exposed by GCC DSI. So, the order of
probes for this to work is GCC->DSI PLL->GCC_MDSS_DSI->DSI controller.

Change-Id: Iee6f1f6db3e52d434d2066374b504a9dc6630e22
Signed-off-by: Taniya Das <tdas@codeaurora.org>