msm: event_timer: Support to add event timers.

Add support to allow drivers to specify future events
that need to be treated as hard deadlines. Upon event timer
expiry the driver will be notified by callback function
that the driver provides.
The pm code can query the earliest event and modify
the event timer accounting for the back off based on the
latency of the low power mode being entered.

Change-Id: Iec26f77de3b826bff456b12f3230268d9bdc2255
Signed-off-by: Girish Mahadevan <girishm@codeaurora.org>