stmmac: support Reg_9 to get HW level information

For GMAC newer than 3.40a there is a new register (Reg_9) that provides the
status of all modules of the transmit and receive paths and FIFO status.
These can be exposed via ethtool.

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