spm: use virtual counter for sp_sleep

SPs(at S-EL1) can only access virtual counters which is calculated as:
  Virtual Count = Physical Count - <offset>
Offset value is specified in the register CNTVOFF_EL2, which is set
as 0 by hafnium.

This patch changes sp_sleep() to use virtual counter instead of
physical counter.

Change-Id: I4245b5efbc0a982686e8fd22060110faa764a729
Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
2 files changed