drm/msm/dp: add debugfs node to set/get max pclk

Add debug fs node, max_pclk_khz, to configure the maximum PCLK
supported. The maximum PCLK value limits the maximum resolution
and fps supported for the connected external display.

Example: To set 650MHz PCLK: echo 650000 > /d/drm_dp/max_pclk_khz

CRs-Fixed: 2252931
Change-Id: If17989616e31ed44256affee120917b92ef892e9
Signed-off-by: Govinda Rajulu Chenna <gchenna@codeaurora.org>