stmmac: prepare stmmac platform to support stand alone drivers

Prepare the stmmac platform code to support standalone drivers
by exporting the need functions and having of_match_device use
the match table reference already present in the driver struct.

This will allow us to reuse the platform driver functions from
this code easily in other stand alone platform drivers.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>