MFC r324495: 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.
NOTE: r326311's changes to arm64/efirt_machdep.c have been hand-applied to
this import of efirt_machdep.c due to r326311 having already been merged.