dt/net: add helper function of_get_phy_mode

It adds the helper function of_get_phy_mode getting phy interface
from device tree.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Cc: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: David Miller <davem@davemloft.net>
diff --git a/include/linux/of_net.h b/include/linux/of_net.h
index e913081..f474641 100644
--- a/include/linux/of_net.h
+++ b/include/linux/of_net.h
@@ -9,6 +9,7 @@
 
 #ifdef CONFIG_OF_NET
 #include <linux/of.h>
+extern const int of_get_phy_mode(struct device_node *np);
 extern const void *of_get_mac_address(struct device_node *np);
 #endif