kboot: aarch64 memory enumeration enumerate_memory_arch()

We have an odd situation with aarch64 memory enumeration. The fdt that
we can get has a PA of the UEFI memory map, as modified by the current
running Linux kernel so it can retain those pages it needs for EFI and
other services. We have to pass in this EFI tablem but don't have access
to it in the boot loader. We do in the trampoline code, so a forthcoming
commit will copy it there for the kernel to use. All for want of /dev/mem
in the target environment sometimes.

However, we also have to find a place to load the kernel, so we have to
fallback to /proc/iomem when we can't read the UEFI memory map directly
from /dev/mem. It will give us good enough results to do this task. This
table isn't quite suitable to be converted to the EFI table, so we use
both methods. We'll fall back to this method also if there's no EFI
table advertised in the fdt. There's no /sys file on aarch64 that has
this information, hence using the old-style /proc/iomem. We're unlikely
to work if there's no EFI, though.

Note: The underlying Linux mechanism is different than the amd64 method
which seems like it should be MI, but unimplemented on aarch64.

Sponsored by: Netflix
Discussed with: kevans
Differential Revision: https://reviews.freebsd.org/D38249