drm/msm/dp: add debugfs node for max_lclk_khz

Add debugfs node to read/write max_lclk_khz to validate
different link clock configurations dynamically with the
device that supports more than one link clock frequency.

CRs-Fixed: 2372619
Change-Id: I11b7860e601fa087e7fd31894fb9cec379bdf639
Signed-off-by: Govinda Rajulu Chenna <gchenna@codeaurora.org>