riscv: MMU detection

Detect and report the supported MMU for each CPU. Export the
capabilities to the rest of the kernel and use it in pmap_bootstrap() to
check for Sv48 support.

Reviewed by: markj
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39814

(cherry picked from commit 7245ffd10eda4ff604840350943d762f70657983)