defconfig: msm8974: Activate System Monitor communication

The System Monitor driver allows a subsystem to be notified
about events on other subsystems, such as 'powering up' or
'shutting down'. Activate this feature for the 8974.

Change-Id: I04b46275ffde083afc344fce9392e8f109f1c317
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>