dt-bindings: drm: msm: add the link clk property information

Add information about the DT property specifying the
maximum DP link clock for a platform.

Change-Id: I91c7fa6eabd047666cb121d25b25985e82d0b769
Signed-off-by: Sankeerth Billakanti <sbillaka@codeaurora.org>
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>