net/macb: bindings doc: add sama5d2 compatibility sting

Add sama5d2 to the biding documentation for this use of the GEM IP.
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/macb.txt

@@ -7,6 +7,7 @@ Required properties:
   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
   the Cadence GEM, or the generic form: "cdns,gem".
+  Use "atmel,sama5d2-gem" for the GEM IP (10/100) available on Atmel sama5d2 SoCs.
   Use "atmel,sama5d3-gem" for the Gigabit IP available on Atmel sama5d3 SoCs.
   Use "atmel,sama5d4-gem" for the GEM IP (10/100) available on Atmel sama5d4 SoCs.
   Use "cdns,zynqmp-gem" for Zynq Ultrascale+ MPSoC.