ofw_cpu: fix __riscv preprocessor check

The canonical name is riscv, not riscv__. Newer compilers no longer
emit the latter.

This re-enables finding the nominal frequency from the CPU's clock.

I checked, and there are no remaining mistakes like this in the tree.

Reviewed by: jrtc27, imp, jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48122