riscv: Include spibus and spigen in GENERIC

We already attempt to enable the SiFive SPI controller, but since spibus
isn't enabled it isn't actually built.

Reviewed by: kp, philip
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31027

(cherry picked from commit 8c439847f0d33fdb79f2bbdced4c300a620d74f5)