drm/msm/dsi-staging: add support to configure dsi clock

Add support to override the DSI byte clock rate for a panel.
This may be needed to satisfy certain performance
requirements where data pixels need to be transmitted within
a specific time period.

Change-Id: I82b9a764f40ce8ce5217d5abca1564df27929eed
Signed-off-by: Vara Reddy <varar@codeaurora.org>