stmmac: rework the code to get the Synopsys ID (v3)

The Synopsys ID is now passed from the MAC core
to the main. This info will be used for managing
the HW cap register (supported in the new GMAC
generations).

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