riscv: add SBI system reset extension
The System Reset extension provides functions to shutdown or reboot the
system via SBI firmware. This newly defined extension supersedes the
functionality of the legacy shutdown extension.
Update the SBI code to use the new System Reset extension when
available, and fall back to the legacy one.
Reviewed By: kp, jhb
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D28226