dt-bindings: arm64: Add BCM4912 SoC to binding document

Add BCM4912 SoC device tree description to bcmbca binding document.
Signed-off-by: William Zhang <william.zhang@broadcom.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

Documentation/devicetree/bindings/arm/bcm/brcm,bcmbca.yaml

@@ -28,6 +28,13 @@ properties:
           - const: brcm,bcm47622
           - const: brcm,bcmbca
 
+      - description: BCM4912 based boards
+        items:
+          - enum:
+              - brcm,bcm94912
+          - const: brcm,bcm4912
+          - const: brcm,bcmbca
+
       - description: BCM63158 based boards
         items:
           - enum: