drivers: net: stmmac: add port selection programming

In case of SGMII more, for example when a MAC2MAC connection
is needed, the port selection bits (inside the MAC configuration
registers) have to be programmed according to the link selected.
So the patch adds a new DT parameter to pass the port selection
and to programmed related PCS and CORE to use it.

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