riscv: Include syscon_power device driver in GENERIC kernel config

[jrtc27]

Summary

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.

Test Plan

Running reboot on QEMU causes the system to reboot, rather than falling back
on cpu_reset which on RISC-V just powers off via SBI.

[br]

sys/riscv/riscv/riscv_syscon.c

4 ↗

(On Diff #74654)

this copyright is not needed

[jrtc27]

sys/riscv/riscv/riscv_syscon.c

4 ↗

(On Diff #74654)

But aw_syscon is copyright kevans@, which is where most of this code comes from? As in, I copied that file, changed aw to riscv everywhere, updated compat_data, the device description and comments. But it's basically the same code.

[br]

sys/riscv/riscv/riscv_syscon.c

4 ↗

(On Diff #74654)

Since you copied then yes, otherwise it looks like a basic FDT driver template