net: macb: add support for mii on rgmii

Cadence IP has option to enable MII support on RGMII interface. This
could be selected though bit 28 of network control register. This option
is not enabled on all the IP versions thus add a software capability to
be selected by the proper implementation of this IP.

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