stmmac: move stmmac_pltfr_probe into dwmac-generic

As all dwmac-* drivers now have their own probe function move
the common one into dwmac-generic driver and drop the EXPORT.

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