riscv: reject CPUs with mmu-type "riscv,none"

According to riscv/cpus.yaml in the device-tree docs, this property may
exist but indicate that the CPU does not have an MMU. Detect this
possibility.

Reviewed by: jhb
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36980