msm: sysmon: Introduce sysmon_get_reason() for querying failure reasons

The sysmon_get_reason() API queries a subsystem that has crashed for
the reason of the failure and returns it as string.  Common code between
this new API and sysmon_send_event() is refactored into sysmon_send_msg().

Change-Id: I1e4fda7794f493ee286346ade7b1e779a43d1e4c
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>