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

[jrtc27]

Summary

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.

[imp]

This is a low-overhead way to enable graphics cards, so I'm happy with it.