xen/arm64: mm: Add memory to the boot allocator first

xen/arm64: mm: Add memory to the boot allocator first

Currently, memory is added to the boot allocator after the xenheap
mappings are done. This will break if the first mapping is more than
512GB of RAM.

In addition to that, a follow-up patch will rework setup_xenheap_mappings()
to use smaller mappings (e.g. 2MB, 4KB). So it will be necessary to have
memory in the boot allocator earlier.

Only free memory (e.g. not reserved or modules) can be added to the boot
allocator. It might be possible that some regions (including the first
one) will have no free memory.

So we need to add all the free memory to the boot allocator first
and then add do the mappings.

Populating the boot allocator is nearly the same between arm32 and
arm64. The only difference is on the former we need to exclude the
xenheap for the boot allocator. Gate the difference with CONFIG_ARM_32
so the code be re-used on arm64.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>
Tested-by: Luca Fancellu <luca.fancellu@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>