dt-bindings: add documentation for sama7g5 gigabit ethernet interface

Add documentation for SAMA7G5 gigabit ethernet interface.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/macb.txt

@@ -17,6 +17,7 @@ Required properties:
   Use "cdns,zynqmp-gem" for Zynq Ultrascale+ MPSoC.
   Use "sifive,fu540-c000-gem" for SiFive FU540-C000 SoC.
   Use "microchip,sama7g5-emac" for Microchip SAMA7G5 ethernet interface.
+  Use "microchip,sama7g5-gem" for Microchip SAMA7G5 gigabit ethernet interface.
   Or the generic form: "cdns,emac".
 - reg: Address and length of the register set for the device
 	For "sifive,fu540-c000-gem", second range is required to specify the