msm: camera2: Add monotonic timestamp for SOF

In Camera3, SOF (start of frame) event indicates the time imager
starts to expose. Its timestamp is used for frameworks to do AV
sync for video recording, which needs to use kernel monotonic
clock source. Hence add a new field in the event structure to pass
this along.

Change-Id: I35760bee9a796f058d3f58d1d0532070aff6d2ae
Signed-off-by: Shuzhen Wang <shuzhenw@codeaurora.org>