cpufreq: add option for setting the cpu max and min frequencies

This adds a NixOS option for setting the CPU max and min frequencies
with `cpufreq`.  The two options that have been added are:

- `powerManagement.cpufreq.max`
- `powerManagement.cpufreq.min`

It also adds an alias to the `powerManagement.cpuFreqGovernor` option as
`powerManagement.cpufreq.governor`.  This updates the installer to use
the new option name.  It also updates the manual with a note about
the new name.