xen/arm: mm: Reduce the area that xen_second covers

xen/arm: mm: Reduce the area that xen_second covers

At the moment, xen_second is used to cover the first 2GB of the
virtual address space. With the recent rework of the page-tables,
only the first 1GB region (where Xen resides) is effectively used.

In addition to that, I would like to reshuffle the memory layout.
So Xen mappings may not be anymore in the first 2GB of the virtual
address space.

Therefore, rework xen_second so it only covers the 1GB region where
Xen will reside.

With this change, xen_second doesn't cover anymore the xenheap area
on arm32. So, we first need to add memory to the boot allocator before
setting up the xenheap mappings.

Take the opportunity to update the comments on top of xen_fixmap and
xen_xenmap.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>