net/macb: bindings doc: add sam9x60 binding

Add the compatibility sting documentation for sam9x60 10/100 interface.
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/macb.txt

@@ -4,6 +4,7 @@ Required properties:
 - compatible: Should be "cdns,[<chip>-]{macb|gem}"
   Use "cdns,at91rm9200-emac" Atmel at91rm9200 SoC.
   Use "cdns,at91sam9260-macb" for Atmel at91sam9 SoCs.
+  Use "cdns,sam9x60-macb" for Microchip sam9x60 SoC.
   Use "cdns,np4-macb" for NP4 SoC devices.
   Use "cdns,at32ap7000-macb" for other 10/100 usage or use the generic form: "cdns,macb".
   Use "cdns,pc302-gem" for Picochip picoXcell pc302 and later devices based on