stmmac: initial support to manage pcs modes

This patch adds the minimal support to manage the PCS
modes (RGMII/SGMII) and restart the ANE.
Both TBI and RTBI are not yet supported.

Thanks to Byungho that wrote some part of this code
and tested SGMII too.

The only thing to be fixed is the get/set pause in
ethtool.

Signed-off-by: Byungho An <bh74.an@samsung.com>
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>