Add support for HSM SBI extension

[mhorne]

Summary

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 will be present in
the next release.

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

[br]

sys/riscv/riscv/sbi.c

127

Insert an empty line if the function has no local variables.