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 <rafal@milecki.pl>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210106205838.10964-2-zajec5@gmail.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

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

@@ -15,6 +15,7 @@ maintainers:
 properties:
   compatible:
     enum:
+      - brcm,bcm4908-usb-phy
       - brcm,bcm7211-usb-phy
       - brcm,bcm7216-usb-phy
       - brcm,brcmstb-usb-phy
@@ -113,7 +114,9 @@ allOf:
       properties:
         compatible:
           contains:
-            const: brcm,brcmstb-usb-phy
+            enum:
+              - const: brcm,bcm4908-usb-phy
+              - const: brcm,brcmstb-usb-phy
     then:
       properties:
         reg: