arm64/RPI: enable powerd by default on arm64-aarch64-RPI images

Most 64-bit Raspberry Pi models have a variable processor clock
speed that defaults to a slow speed (e.g. 600 MHz for a nominal
1.5 GHz clock). This results in everything running slowly unless
or until powerd is started, and FreeBSD is then thought to be slow.
Enable powerd by default in /etc/rc.conf on the arm64-aarch64-RPI
images. Tested on Raspberry Pi 3B+ and 4B so far.

PR: 256836
MFC after: 1 month
Reviewed by: rgrimes
Differential Revision: https://reviews.freebsd.org/D43296