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>