drivers: net: cpsw: add support to show hw stats via ethtool

Add support to show CPSW hardware statistics to user via ethtool
so user can find if there were any error reported by hardware or
the system is over loaded duing high data rate transfer.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>