dt-bindings: mailbox: sprd: add compatible for UMS9230

Add a compatible string for the mailbox controller found in the UMS9230
SoC.

Signed-off-by: Otto Pflüger <otto.pflueger@abscue.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>

Author: affenull2345

Documentation/devicetree/bindings/mailbox/sprd-mailbox.yaml

@@ -16,6 +16,7 @@ properties:
     enum:
       - sprd,sc9860-mailbox
       - sprd,sc9863a-mailbox
+      - sprd,ums9230-mailbox
 
   reg:
     items: