clk: qcom: mdss: add DSI PLL clock driver support for msm8909

Add changes for DSI PLL clock driver support for msm8909. Add
the compatibility string of the DSI PLL handle so that the
detection and support of DSI PLL driver for 8909 happens
dynamically. update documentation of clocks property.

Change-Id: If1fb96982433f90c5b82dda8686b7284825bcd09
Signed-off-by: Shivaraj Shetty <shivaraj@codeaurora.org>