rtc: use ktime_get_real_ts64() instead of getnstimeofday64()

getnstimeofday64() is just a wrapper around the ktime accessor, so
we should use that directly.

I considered using ktime_get_boottime_ts64() (to avoid leap second
problems) or ktime_get_real_seconds() (to simplify the calculation,
but in the end concluded that the existing interface is probably
the most appropriate in this case.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>