stmmac: parameters auto-tuning through HW cap reg

New GMAC devices (newer than the databook 3.50a) have the
HW capability register that provides which features are actually
supported by the hardware.

On old devices many information have to be passed through the
platform, for example: enhanced descriptor structure,
TX COE etc. These are mandatory to properly configure the driver.
This remains still valid because the driver has to support old
Synopsys devices but now it's also able to override them using the
values from the HW capability register if supported.

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