ARM: dts: msm: Add node for mpm sleep counter for sdxprairie

Add device node for mpm2 sleep counter which is required
for the boot stats KPI reporting on sdxprairie target.

Change-Id: Ie89156eeec9539643faa1cc43d48d4fbe5393f94
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>