net: stmmac: Allocate and pass soc/board specific data to callbacks

The current .init and .exit callbacks requires access to driver
private data structures. This is not a good seperation and abstraction.

Instead, we add a new .setup callback for allocating private data, and
pass the returned pointer to the other callbacks.

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