Add a driver for the EFI RTC. This uses the EFI Runtime Services to query
the system time.

As we seem to only read this time on boot, and this is the only source of
time on many arm64 machines we need to enable this by default there. As
this is not always the case with U-Boot firmware, or when we have been
booted from a non-UEFI environment we only enable the device driver when
the Runtime Services are present and reading the time doesn't result in an
error.

PR: 212185
Reviewed by: imp, kib
Tested by: emaste
Relnotes: yes
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D12650