net: macb: enable mii on rgmii for sama7g5

Both MAC IPs available on SAMA7G5 support MII on RGMII feature.
Enable these by adding proper capability to proper macb_config
objects.

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