net: netcp: ethss: make sgmii configuration conditional

As a preparatory patch to add support for 2u cpsw hardware found on
K2G SoC, make sgmii configuration conditional. This is required
since 2u uses RGMII interface instead of SGMII and to allow for driver
re-use.

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