riscv: tweak SoC-specific conf organization

Hide some lines from the main GENERIC files by mimicking arm64's model.

I do not have any intention of creating a std.riscv or SIFIVE
configuration file at this time.

Reviewed by: jrtc27
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42910