ARM: dts: msm: Add RTB trace support for 8939

Add rtb support to device tree node which enables RTB logging on msm8939.
RTB logging provides useful debug information by logging events to
uncached buffer which can be processed from ramdumps in case of device
reset.

Change-Id: Iff24922b526bbce405cff84bfdf059f5f09ab9d1
Signed-off-by: Xiaogang Cui <xiaogang@codeaurora.org>