enabling powerd on RPi

I am looking at enabling powerd by default on the Raspberry Pi 4 and maybe
others.  There is a bug from 2021 on the subject which has gotten some recent
discussion, https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=256836.  Also,
problems come up from time to time about performance problems because people
don't know to enable powerd.  It makes FreeBSD look much slower than Linux.

The simplest action is to enable powerd by default on the arm64-aarch64-RPI
images.  This would affect RPi 4 and variants, also RPI 3* and later RPi 2.
I enabled powerd on an RPi 3B+, and it seems to have no issues; it seems
to work.  Does anyone know of a disadvantage of enabling powerd on RPI
images for all targets?  The alternative would be to configure at the first
boot, although I'm not positive of a definitive way to identify the RPi
variants.  Maybe just looking for a dev.cpu.0.freq sysctl node would
suffice.

If no one objects, I will make changes to enable powerd on RPI snapshots
for 15-current, and we can see what happens.

		Mike