RISC-V: build SiFive drivers and DTB in GENERIC

[mhorne]

Summary

In the spirit of the GENERIC config, we should include the drivers required to
run on most supported platforms.

Test Plan

Build GENERIC.

[kp]

sys/riscv/conf/GENERIC

171

The always on hardware exists only on the FE310, not the FU540 chip.
We don't support running on FE310 hardware (it's rv32 for one), so I don't know if we want to build that driver in.

It exists because there's other RISC-V RV64 hardware that does have the aon device, but you can't buy that (yet).

[mhorne]

Remove fe310 always-on device.