drm/msm/dsi-staging: add XO clock support for DSI display

Add optional xo_clk in DSI display dtsi node to support XO shutdown
hibernation to disk. When XO clock is defined, select XO clock src
when DSI is disabled.

Change-Id: Ibc5c8b945cdcce179b00c02cf06412984ffc1a2c
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>