xen/arm: Warn when a device tree path will be re-used by Xen

xen/arm: Warn when a device tree path will be re-used by Xen

Xen is unconditionally using certain device tree paths to create DOM0
specific node (for instance /psci, /memory and /hypervisor).

Print a warning message on the console to let the user know if we
re-use one of these nodes.

Note that the content of most of those is very common and they
should have already been skipped via the compatible string or type
string. This warning is here to catch unusual device-tree and
compatible string that we may not yet support in Xen.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>