power: supply: ab8500: stop using getnstimeofday64()
getnstimeofday64() is deprecated in favor of the ktime_get() family.
The direct replacement would be ktime_get_real_ts64(), but we only need
the seconds value, and it seems better to use boottime than real time
to avoid unexpected behavior with a concurrent settimeofday().
ktime_get_seconds() might also work, but it seems better to use
boottime than monotonic time since I assume that the charging
process continues during suspend.