net: netcp: ethss: add support for handling rgmii link interface

2u cpsw hardware on K2G uses rgmii link to interface with Phy. So add
support for this interface in the code so that driver can be re-used
for 2u hardware.

Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>