arm64: mm: Log the process id in the rtb

Store the process id in the register trace buffer.
This gives a general idea about what process was last
running when the RTB stopped.

Change-Id: Idcbd8db9eb451f2d732c935f00b6c50f9af3c592
Signed-off-by: Se Wang (Patrick) Oh <sewango@codeaurora.org>