MFC r331068:

Use EFI RTC capabilities info when registering, add bootverbose diagnostics.

Make some small improvements to the efirtc driver by obtaining the clock
capabilities (resolution and whether the sub-second counters are reset) and
using the info when registering the clock. When the hardware zeroes out the
subsecond info on clock-set, schedule clock updates to happen just before
top-of-second, so that the RTC time is closely in-sync with kernel time.

Also, in the identify() routine, always add the driver if EFI runtime
services are available, then decide in probe() whether to attach the driver
or not. If not attaching and bootverbose is on, say why. All of this is
basically to avoid "silent failure" -- if someone thinks there should be an
efi rtc and it's not attaching, at least they can set bootverbose and maybe
get a clue from the output.

Differential Revision: https://reviews.freebsd.org/D14565 (timed out)