dt-bindings: net: cdns,macb: document polarfire soc's macb

Until now the PolarFire SoC (MPFS) has been using the generic
"cdns,macb" compatible but has optional reset support. Add a specific
compatible which falls back to the currently used generic binding.
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/devicetree/bindings/net/cdns,macb.yaml

@@ -28,6 +28,7 @@ properties:
           - enum:
               - cdns,at91sam9260-macb # Atmel at91sam9 SoCs
               - cdns,sam9x60-macb     # Microchip sam9x60 SoC
+              - microchip,mpfs-macb   # Microchip PolarFire SoC
           - const: cdns,macb          # Generic
 
       - items: