arm64: Add support for the RK805/RK808 RTC

Implement a driver for the RTC embedded in the RK805/RK808 power
management system used for RK3328 and RK3399 SoCs.

Based on experiments on my RK808, setting the time doesn't alter the
internal/inaccessible sub-second counter, therefore there's no point
in calling clock_schedule().

Based on an earlier revision by andrew.

Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D22692
Sponsored by: Google
MFC after: 1 week