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

Add support to device tree node to enable RTB logging on samarium.
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: I77795826a80d63103b41e65aacaf3f1e6eeb8e6d
Signed-off-by: Aparna Das <adas@codeaurora.org>