ARM: dts: msm: Add system stats driver for msm8996

The system stats driver provides the statistics about system low power
modes and the respective masters' votes into system modes. This is a merge
of rpm-stats and rpm-master-stats modules. The rpm-stats and
rpm-master-stats driver would be eventually deprecated in preference to the
system stats driver.

Change-Id: I8a8842250fe459cb523b607ca379f9a036b4c09b
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>