arm/dt: msm8974: Update the start address of PC counters

Update the PC counter address to Debug IMEM region. On 8974, the IMEM
region doesn't hold its value through reset. The updated IMEM debug
region is in RPMs message RAM.

Change-Id: I692edaa262411c44c15fdd32fa15a798afda3a18
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>