net: ibm_newemac: convert it to use of_get_phy_mode

The patch extends 'enum phy_interface_t' and of_get_phy_mode a little
bit with PHY_INTERFACE_MODE_NA and PHY_INTERFACE_MODE_SMII added,
and then converts ibm_newemac net driver to use of_get_phy_mode
getting phy mode from device tree.

It also resolves the namespace conflict on phy_read/write between
common mdiobus interface and ibm_newemac private one.

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 Miller <davem@davemloft.net>