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>