stmmac: add proper probe function to dwmac-lpc18xx

By using a few functions from stmmac_platform we can now create
a proper probe function in this driver. By doing so we can drop
the OF match data and simplify the overall driver.

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