ARM: dts: msm: Add dts entry mpm2-sleep-counter for MSMCOBALT

Add dts mpm2-sleep-counter node which is used by the boot_stats driver.
This dt node gives imem offset to read boot stats written by the
bootloader.

CRs-Fixed: 1037982
Change-Id: I4f8ec495ae02908d1d8cd7e86148a6d4bd38b198
Signed-off-by: Runmin Wang <runminw@codeaurora.org>