conf: deduplicate dwc config logic

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.

Reviewed by: manu
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D44103