riscv: Port improvements from arm64/amd64 pmaps, part 2

• Give pmap_promote_l2() a return value indicating whether or not promotion succeeded.
• Check pmap_ps_enabled() in pmap_promote_l2() rather than making callers do it.
• Annotate superpages_enabled with __read_frequently.

Reviewed by: kib
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D42289