Support the EFI Runtime Services on arm64. As with amd64 we use the 1:1
mapping. This uses the new common code shared with amd64.
The RTC should only be accessed via EFI. There is no locking around it as
the spec only has this as a requirement for the PC-AT CMOS device.
Reviewed by: kib, imp
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D12595