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

arm64: Add per SoC family kernel config

There is multiple reason for this :
- This makes it easier to see which driver is needed for each SoC
- This makes it easier to create a custom config for one SoC
- This really reduce boot time (which some people might want)

Some explaination about the files :
- std.arm64 contains all standard kernel option
- std.dev contains all the standard kernel devices
- std.<soc> contains all drivers needed to boot on this SoC family
- <SOC> includes std.arm64, std.dev and std.<soc>
- GENERIC includes std.arm64, std.dev and all std.<soc>

Sponsored by: Diablotin Systems
MFC After: 2 months
Reviewed by: mmel, cognet, imp
Differential Revision: https://reviews.freebsd.org/D30474