rpm_stats: Update the reg offset for rpm-stats

The base address has changed and not fixed anymore. Read the offset
from the image header and update the base address based on the offset.

Change-Id: I5f51fc320666e4ff16f86735614413f54571bc1c
Signed-off-by: Archana Sathyakumar <asathyak@codeaurora.org>