net/smsc911x: add device tree probe support

It adds device tree probe support for smsc911x driver.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Steve Glendinning <steve.glendinning@smsc.com>
Cc: David S. Miller <davem@davemloft.net>
Reviewed-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>