riscv: add some more drivers to GENERIC

Enable phy and regulator extres devices. These aren't needed for
existing SoC support, but are of general utility to FDT platforms and
enable out-of-tree work.

Similarly, enable sdhci and mmc.

Reviewed by: jrtc27
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42913