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

Documentation/devicetree/bindings/vendor-prefixes.yaml

@@ -1368,6 +1368,8 @@ patternProperties:
     description: Wanchanglong Electronics Technology（SHENZHEN）Co.，Ltd.
   "^x-powers,.*":
     description: X-Powers
+  "^xen,.*":
+    description: Xen Hypervisor
   "^xes,.*":
     description: Extreme Engineering Solutions (X-ES)
   "^xiaomi,.*":