stmmac: dwmac-sti: use custom of match structure

Create a new private structure for OF match data in the
dwmac-sti driver. This enables us to eventually drop the
common OF match data structure which contains a lot of
unused fields.

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