ARM: dts: msm: add xo_clk for DP display on sm8150

During suspend to disk we need to set DP PCLK to xo during
disable. This changes adds an xo_clk entry to DP device tree node.

Change-Id: Id9dd3cf04e273484be4484d20fedd618c9e119a8
Acked-by: Poojashree Masthi <c_pmasth@qti.qualcomm.com>
Signed-off-by: Rahul Sharma <rahsha@codeaurora.org>