stmmac: add dwmac glue for NXP 18xx/43xx family

Add support for Ethernet on NXP LPC18xx and LPC43xx using the
dwmac driver. This glue is required to setup phy interface
mode, MII or RMII, on the SoC.

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