dt-bindings: arm: Add BCM6878 soc

Add BCM6878 SOC device tree description to bcmbca binding document.
Signed-off-by: Anand Gore <anand.gore@broadcom.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

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

@@ -56,6 +56,13 @@ properties:
           - const: brcm,bcm6858
           - const: brcm,bcmbca
 
+      - description: BCM6878 based boards
+        items:
+          - enum:
+              - brcm,bcm96878
+          - const: brcm,bcm6878
+          - const: brcm,bcmbca
+
 additionalProperties: true
 
 ...