rpm_logs: Update the reg offset for rpm-logs

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

Change-Id: I5084d9932e312593bb16a9cbe3dd2b42d920d89e
Signed-off-by: Archana Sathyakumar <asathyak@codeaurora.org>