xen/arm32: setup: Give a xenheap page to the boot allocator

xen/arm32: setup: Give a xenheap page to the boot allocator

After commit 6e3e771203 "xen/arm: setup: Relocate the Device-Tree later on
in the boot", the boot allocator will not receive any xenheap page (i.e.
mapped page) on Arm32.

However, the boot allocator implicitly relies on having the first page
already mapped and therefore result to break boot on Arm32.

The easiest way for now is to give a xenheap page to the boot allocator.
We may want to rethink the interface in the future.

[stefano: fix grammar in commit message]

Fixes: 6e3e771203 ('xen/arm: setup: Relocate the Device-Tree later on in the boot')
Signed-off-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>