net: stmmac: Use driver data and callbacks tied with compatible strings

The stmmac driver core allows passing feature flags and callbacks via
platform data. Add a similar stmmac_of_data to pass flags and callbacks
tied to compatible strings. This allows us to extend stmmac with glue
layers for different SoCs.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: David S. Miller <davem@davemloft.net>