Enable RPi's CPU Frequency scaling. Fixes bug 991921.

The first commit makes CPU Frequency scaling work at all.

The second commit changes the default governor to 'ondemand'. This is different from 'schedutil' used by RPi 2/3/4 because CONFIG_SMP isn't enabled for RPi 0/0w/1. After these changes, cpupower frequency-info shows useful information for 0/0w/1. The RPi 1 only has 1 frequency, but the 0 and 0w also allow you or the governor to change the CPU frequency.

The third commit is optional, but I only learned by creating this MR that you/one can extend the list of available governors by modprobe-ing cpufreq governor kernel modules. Previously I always thought that other governors weren't available at all.
As this only affect RPi 0/0w/1 devices, I thought it was an acceptable trade-off. Feel free to leave that out if you disagree.

I also request that these changes will be considered for (a point release of) Bullseye, so that RPi 0/0w/1 users running Bullseye will be able to use their devices more energy efficient (in the future).