conf: deduplicate dwmmc config logic

The core of this driver is supported by multiple architectures. Move the
config entries to the MI conf/files.

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/D44104