Add support for HSM SBI extension

The Hardware State Management (HSM) extension provides a set of SBI
calls that allow the supervisor software to start and stop hart
execution.

The HSM extension has been implemented in OpenSBI and is present in
the v0.7 release.

[1] https://github.com/riscv/riscv-sbi-doc/blob/master/riscv-sbi.adoc

Reviewed by: br
Differential Revision: https://reviews.freebsd.org/D24496