riscv: enable EFI framebuffer

[mhorne]

Summary

Pass framebuffer information from loader(8) to the kernel via the
MODINFOMD_EFI_FB metadata field.

Enable the vt_efifb driver. A small tweak is required to work around the
lack of VM_MEMATTR_WRITE_COMBINING on this platform; we use
VM_MEMATTR_UNCACHEABLE instead.

Test Plan

Tested with the RVVM simulator. We are missing input drivers required for the graphical mode, but the framebuffer works.