net: ethernet: stmmac: update MDIO support for GMAC4

On new GMAC4 IP, MAC_MDIO_address register has been updated, and bitmaps
changed. This patch takes into account those changes.

Signed-off-by: Alexandre TORGUE <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>