net: ethernet: ti: cpsw: move ethtool func in separate file

As a preparatory patch to add support for a switchdev based cpsw driver,
move common ethtool functions to separate cpsw-ethtool.c file so that they
can be used across both drivers. It will simplify CPSW driver code
maintenance also.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>