arm64/machdep: Reserve memory when we find Linux EFI reserved memory table

When Linux loads a new kernel via kexec, somtiems it must reserve memory
for devices that are still active (and typically can't be reset or
shutdown). When present, this table is a linked list of ranges that are
still in use that the OS must avoid using.

Mark these areas as reserved.

This is part of the GICv3 workaround code where we must use the PA
addresses already programmed into the GICv3 when we take over. This part
ensure we don't allocate the mmeory for anything else.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D37440