net/fec: add device tree probe support

It adds device tree probe support for fec driver.

Signed-off-by: Jason Liu <jason.hui@linaro.org>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Cc: David S. Miller <davem@davemloft.net>
Cc: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: David S. Miller <davem@davemloft.net>