Generic secure time layer

- Moved platform-dependent code out of tee_time.c to make it a generic
  time layer.
- Added an abstract layer for platforms to implement their own secure
  time source.
- Implemented arm cntpct as one of secure time source.
- Moved rtt0 related time operation from tee_time.c to tee_time_rtt.c,
  act as another secure time source.
- Added tee_time_ree.c for the plaform that doesn't have secure time
  source. In this case, using ree time as secure time source.