Add a new RTC driver for Armada38x

New driver registers RTC as system clock. New RTC resolution is 1 sec.
Settime and gettime functions were implemented.

Reviewed by: andrew, ian
Obtained from: Semihalf
Sponsored by: Stormshield
Submitted by: Jan Dabros <jsd@semihalf.com>
Differential revision: https://reviews.freebsd.org/D4421