dt-bindings: usb: xhci: Add Broadcom STB v2 compatible device

For some reason the "brcm,xhci-brcm-v2" compatible string has been missing
in the original bindings file. Add it to the Generic xHCI Controllers DT
schema since the controller driver expects it to be supported.

Reviewed-by: Rob Herring <[e-mail]>
Acked-by: Florian Fainelli <[e-mail]>
Signed-off-by: Serge Semin <[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

···
               - marvell,armada-8k-xhci
           - const: generic-xhci
       - description: Broadcom STB SoCs with xHCI
-        const: brcm,bcm7445-xhci
+        enum:
+          - brcm,xhci-brcm-v2
+          - brcm,bcm7445-xhci
       - description: Generic xHCI device
         const: xhci-platform
         deprecated: true