net: ethernet: stmmac: add support of Synopsys 3.50a MAC IP

Adds support of Synopsys 3.50a MAC IP in stmmac driver.

Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Tested-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Alexandre TORGUE <alexandre.torgue@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>