Exclude the EFI framebuffer from phys_avail[] on arm64.

On the ThunderX the region occupied by the framebuffer is included in
the EFI map, so explicitly add it to the set of regions that aren't
managed by the physical memory allocator.

PR: 231064
Reviewed by: andrew
Approved by: re (gjb)
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D17073