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>
diff --git a/include/lib/aarch64/arch_helpers.h b/include/lib/aarch64/arch_helpers.h
index 9bcd0bb..6a96ea2 100644
--- a/include/lib/aarch64/arch_helpers.h
+++ b/include/lib/aarch64/arch_helpers.h
@@ -367,6 +367,7 @@
 DEFINE_SYSREG_RW_FUNCS(cntp_tval_el0)
 DEFINE_SYSREG_RW_FUNCS(cntp_cval_el0)
 DEFINE_SYSREG_READ_FUNC(cntpct_el0)
+DEFINE_SYSREG_READ_FUNC(cntvct_el0)
 DEFINE_SYSREG_RW_FUNCS(cnthctl_el2)
 
 #define get_cntp_ctl_enable(x)  (((x) >> CNTP_CTL_ENABLE_SHIFT) & \