dt-bindings: phy: brcm, brcmstb-usb-phy: add BCM4908 binding

BCM4908 uses the same PHY and may require just a slightly different
programming.

Signed-off-by: Rafał Miłecki <[e-mail]>
Acked-by: Florian Fainelli <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Vinod Koul <[e-mail]>

Documentation/devicetree/bindings/phy/brcm,brcmstb-usb-phy.yaml

···
 properties:
   compatible:
     enum:
+      - brcm,bcm4908-usb-phy
       - brcm,bcm7211-usb-phy
       - brcm,bcm7216-usb-phy
       - brcm,brcmstb-usb-phy
···
       properties:
         compatible:
           contains:
-            const: brcm,brcmstb-usb-phy
+            enum:
+              - const: brcm,bcm4908-usb-phy
+              - const: brcm,brcmstb-usb-phy
     then:
       properties:
         reg: