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>