clk: qcom: mdss: add DSI PLL clock driver support for msm8939

This change adds the DSI PLL clock driver support for
msm8939. Add the compatibility string of the DSI
PLL handle so that the detection and support of DSI
PLL driver for 8939 happens dynamically.

Change-Id: Iaa4be3541ce938816d5b9552b685ce05e7cdab64
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>