drivers: cpuidle: lpm-levels: Fix round off error in calculation

Fix round off error in calculation of wake up time programmed.
Current implementation adds extra delay in wake up time due to
round off error in conversion of usec to SCLK. Use nsec instead.

CRs-fixed: 1081884
Change-Id: Iecb3b06cde79c59c24a0e56ef05c41a5f9b6204e
Signed-off-by: Maulik Shah <mkshah@codeaurora.org>