ARM: dts: msm: enable variable refresh rate for 8937 MTP/CDP

Add DT property to enable variable refresh rate feature on Truly
1080p video mode panels on 8937 MTP and CDP devices. This switches
the fps of the panel by changing the vertical front porch region
keeping the DSI byte and pixel clock rates constant.

Change-Id: I8c48d6d6f6edd4be596703a4dc70b777e4c007dd
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>