riscv: Create a newbus device for the SBI driver

This approach is based on the Arm PSCI driver, though that makes more
extensive use of its softc than we do here. This will be used to extract
the SBI IPI code as a real PIC.

Reviewed by: mhorne, imp
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D35900