stmmac: add MMC support exported via ethtool (v3)

This patch adds the MMC management counters support.
MMC module is an extension of the register address
space and all the hardware counters can be accessed
via ethtoo -S ethX.

Note that, the MMC interrupts remain masked and the logic
to handle this kind of interrupt will be added later (if
actually useful).

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