net: macb: add userio bits as platform configuration

This is necessary for SAMA7G5 as it uses different values for
PHY interface and also introduces hdfctlen bit.

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>