stmmac: enhance mmc counter management

For gmac3, the MMC addr map is: 0x100 - 0x2fc
For gmac4, the MMC addr map is: 0x700 - 0x8fc

So instead of adding 0x600 to the IO address when setup the mmc,
the RMON base address is saved inside the private structure and
then used to manage the counters.

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