riscv: enable cpufreq_dt driver

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.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48124