dt-bindings: vendor-prefixes: add second HiSilicon prefix

There are few boards DTS using "hisi,rst-syscon" property -
undocumented "hisi" prefix. The property will not be changed in DTS to
non-deprecated one, because of compatibility reasons. Add deprecated
"hisi" prefix to silence DT schema warnings.

Cc: Wei Xu <[e-mail]>
Cc: David Heidelberg <[e-mail]>
Signed-off-by: Krzysztof Kozlowski <[e-mail]>
Reviewed-by: David Heidelberg <[e-mail]>
Reviewed-by: Wei Xu <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/devicetree/bindings/vendor-prefixes.yaml

···
     description: Himax Technologies, Inc.
   "^hirschmann,.*":
     description: Hirschmann Automation and Control GmbH
+  "^hisi,.*":
+    description: HiSilicon Limited (deprecated, use hisilicon)
+    deprecated: true
   "^hisilicon,.*":
     description: HiSilicon Limited.
   "^hit,.*":