dt-bindings: Add vendor prefix for Xen hypervisor

Xen is an open source type-1 hypervisor.

Signed-off-by: Oleksii Moisieiev <oleksii_moisieiev@epam.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/aece6fd976980131120456800de3558e1e2308a0.1646639462.git.oleksii_moisieiev@epam.com