riscv: Add vt and kbdmux to GENERIC for video console support

No in-tree drivers are supported for RISC-V (given it supports UEFI we
could enable the EFI framebuffer, but U-Boot has very limited hardware
support and EDK2 remains a work in progress), but drm-kmod exists with
drivers for video cards that can be used with the HiFive Unmatched.

Reviewed by: imp, jhb
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D32001