qcom: sysmon: Replace the use of enums with strings in sysmon API

Currently, the sysmon_send_shutdown and sysmon_get_reason
APIs expect an enum that identifies a subsystem. Instead,
use a string with the name with which these subsystems have
registered with the SSR framework.

Change-Id: Ie57fe2899f585d07dc8d57bed2dbb91f079a3200
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>