ARM: dts: msm: add rtb to device tree for plutonium

Add support to device tree node to enable RTB logging on msm-plutonium.
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: Ice4640f959bf3ade5233de621a85a3027c75d988
Signed-off-by: Aparna Das <adas@codeaurora.org>