riscv: enable cpufreq_dt driver

[mhorne]

Summary

Implement the small amount of MD code required; copied from arm/arm64.

One tweak is made to cpufreq_dt itself: if the opp-shared property is
missing, but there is only one CPU, then we can still attach. This is
relevant for the single-core Allwinner D1.

Test Plan

Tested on the Allwinner D1 and VisionFive v2 (JH7110) SBCs.

CPUs can have their frequency toggled via sysctl, or powerd -a max. I did not verify the effects, but all the machinery appears to be working.