xen/arm: use correct device tree root node name

xen/arm: use correct device tree root node name

The root node of a device tree should not have a node name. This is
specified in section 2.2.1 of version 0.2 of the device tree
specification, available from devicetree.org.

Linux Kernel versions prior to 4.15 misinterpret flattened device trees
with a "/" as the name of the root node as an FDT version older than 16.
Linux then fails to parse the FDT.

Signed-off-by: Will Abele <will.abele@starlab.io>
Reviewed-by: Julien Grall <julien.grall@arm.com>