riscv: Include syscon_power device driver in GENERIC kernel config

QEMU's RISC-V virt machine provides syscon-power and syscon-reset
devices as the means by which to shutdown and reboot. We also need to
ensure that we have attached the syscon_generic device before attaching
any syscon_power devices, and so we introduce a new riscv_syscon device
akin to aw_syscon added in r327936. Currently the SiFive test finisher
is used as the specific implementation of such a syscon device.

Reviewed by: br, brooks (mentor), jhb (mentor)
Approved by: br, brooks (mentor), jhb (mentor)
Obtained from: CheriBSD
Differential Revision: https://reviews.freebsd.org/D25725