dt-bindings: usb: xhci: Add support for BCM2711

The xHCI IP on the BCM2711 SoC is compatible to "brcm,xhci-brcm-v2",
but also requires a power domain. So introduce a new compatible
and the specific constraints. Since the key allOf can only occur
once, merge the reference below.

Signed-off-by: Stefan Wahren <[e-mail]>
Reviewed-by: Conor Dooley <[e-mail]>
Reviewed-by: Florian Fainelli <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/devicetree/bindings/usb/generic-xhci.yaml

···
 maintainers:
   - Mathias Nyman <[e-mail]>
 
-allOf:
-  - $ref: usb-xhci.yaml#
-
 properties:
   compatible:
     oneOf:
···
               - marvell,armada-380-xhci
               - marvell,armada-8k-xhci
           - const: generic-xhci
+      - description: Broadcom SoCs with power domains
+        items:
+          - enum:
+              - brcm,bcm2711-xhci
+          - const: brcm,xhci-brcm-v2
       - description: Broadcom STB SoCs with xHCI
         enum:
           - brcm,xhci-brcm-v2
···
       - const: core
       - const: reg
 
+  power-domains:
+    maxItems: 1
+
 unevaluatedProperties: false
 
 required:
   - compatible
   - reg
   - interrupts
+
+allOf:
+  - $ref: usb-xhci.yaml#
+  - if:
+      properties:
+        compatible:
+          contains:
+            const: brcm,bcm2711-xhci
+    then:
+      required:
+        - power-domains
+    else:
+      properties:
+        power-domains: false
 
 examples:
   - |