conf: deduplicate dwc config logic

[mhorne]

Summary

Move the core dwc(4) file entries to the MI conf/files.
Platform-specific versions e.g. rk_dwc are retained in their current
position. On arm64 this necessitates adding an additional 'device dwc'
option.

This hardware is found on several available/emerging RISC-V SoCs, so we
will soon need it on this architecture. Add it to LINT for now.

[jrtc27]

sys/riscv/conf/NOTES

61

This seems like a separate change?

[mhorne]

sys/riscv/conf/NOTES

61

Yeah, you are right. I will split this out before commit, same goes for dwmmc addition in D44104.

[jrtc27]

sys/riscv/conf/NOTES

61

Ok. I think it's fine to merge the two riscv dwc* changes into one commit though if you want to, though commits are cheap.