ARM: dts: msm: reserve memory for display ramdump for sm6150

Reserve memory for display ramdump feature on sm6150 target.
This memory buffer would be configured by the bootloader and
needs to be retained forever.

Change-Id: I429fb21a0399399f6649e982f54a1b2264b5b6fa
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>