Add support for CPU freq to Raspberry PI 4

[me_jamesrm.com]

Summary

Adds the raspberry pi 4 to the compat data
in the bcm2835_cpufreq driver.

Test Plan

Before this patch running powerd(4) would fail

Afterwards powerd(4) works. Also the output from
sysctl.dev.cpu.0 appears as follows:

dev.cpu.0.temperature: 54.0C
dev.cpu.0.freq_levels: 1500/-1
dev.cpu.0.freq: 1500
dev.cpu.0.%parent: cpulist0
dev.cpu.0.%pnpinfo: name=cpu@0 compat=arm,cortex-a72
dev.cpu.0.%location:
dev.cpu.0.%driver: cpu
dev.cpu.0.%desc: Open Firmware CPU

and the CPU frequency can be controlled via:

sysctl dev.cpu.0.freq=1500

[me_jamesrm.com]

Cleanup

[me_jamesrm.com]

Cleanup

[kevans]

LGTM, values look sane, I can probably commit this tonight.