msm: mdss: dp: add support to parse maximum PCLK from dtsi for display port

For SDM660 and MSM8998 even though the DP controller version is
the same, there is a difference in the maximum pixel clock
frequency supported between these targets. Add support to parse
this property from the MDSS Display Port dtsi node as an optional
property.

Change-Id: I568ac44e41c7bf34f4b87b3199cdecad54e166c3
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>