xen/arm: setup: Relocate the Device-Tree later on in the boot

xen/arm: setup: Relocate the Device-Tree later on in the boot

At the moment, the Device-Tree is relocated into xenheap while setting
up the memory subsystem. This is actually not necessary because the
early mapping is still present and we don't require the virtual address
to be stable until unflatting the Device-Tree.

So the relocation can safely be moved after the memory subsystem is
fully setup. This has the nice advantage to make the relocation common
and let the xenheap allocator decides where to put it.

Lastly, the device-tree is not going to be used for ACPI system. So
there are no need to relocate it and can just be discarded.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>