riscv: T-HEAD PBMT support

T-HEAD CPUs provide a spec-violating implementation of page-based memory
types, using PTE bits [63:59]. Add basic support for this "errata",
referred to in some places as an "extension".

Note that this change is not enough on its own, but a workaround is
needed for the bootstrap (locore) page tables as well.

Reviewed by: jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45472