conf: deduplicate dwmmc config logic

[mhorne]

Summary

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. Add it to LINT for now.

[jrtc27]

sys/riscv/conf/NOTES

49 ↗

(On Diff #135083)

As with dwc, separate?