msm: clk: Add snapshot of Downstream DSI pll driver

This is snapshot of DSI pll clock driver as of msm-3.18
commit 15c54a16fe5c ("Revert msm: mdss: fix NOC error
issue on mdss-pll").

Change-Id: I779e64191a96463c93c68fd97bb1deedc6b121ff
Signed-off-by: Sachin Bhayare <sachin.bhayare@codeaurora.org>