msm: camera: Change Avtimer timestamp implementation

If video telephony app uses camera, timestamp need to be more
accurate. Use hardware registers to read time in nano seconds
and fill it in buffer.

Change-Id: I25c321ec83fb952f09a57b1b0ad46b0458a990a1
Signed-off-by: Abhishek kondaveeti <akondave@codeaurora.org>