drm: xilinx: dp: Add the pixel clock frequency DT property

The supported maximum pixel clock can vary depending on the IP
configuration. This patch adds the optional pclock DT property,
"xlnx,max-pclock-frequency", so the maximum pixel clock rate
can be configured when needed.

Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>