riscv: enable EFI framebuffer

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.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D48884