drivers: clksource: Add API to return cval

Sleep driver needs to program the absolute clock value for next expiry
into pdc register for system wake up. Add an API to read the cval low
and high counters.

Change-Id: I3f08b3320ea281894d46f3c4bb0862784cc337a1
Signed-off-by: Channagoud Kadabi <ckadabi@codeaurora.org>