raspberry pi 4: cpufreq support

The submitter notes that the bcm2835_cpufreq driver really just needs the
rpi4 compat string added to it; powerd subsequently works and the dev.cpu.0
sysctl values look sane and can be successfully manipulated.

Submitted by: James Mintram <me@jamesrm.com>
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D25349