sh_eth: add RGMII support

The R-Car V3H (AKA R8A77980) GEther controller  adds support for the RGMII
PHY interface mode as a new  value  for the RMII_MII register.

Based on the original (and large) patch by Vladimir Barinov.

Signed-off-by: Vladimir Barinov <vladimir.barinov@cogentembedded.com>
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/renesas/sh_eth.c

@@ -466,6 +466,9 @@ static void sh_eth_select_mii(struct net_device *ndev)
 	u32 value;
 
 	switch (mdp->phy_interface) {
+	case PHY_INTERFACE_MODE_RGMII ... PHY_INTERFACE_MODE_RGMII_TXID:
+		value = 0x3;
+		break;
 	case PHY_INTERFACE_MODE_GMII:
 		value = 0x2;
 		break;