net: netcp: ethss: use of_get_phy_mode() to support different RGMII modes

The phy used for K2G allows for internal delays to be added optionally
to the clock circuitry based on board desing. To add this support,
enhance the driver to use of_get_phy_mode() to read the phy-mode from
the phy device and pass the same to phy through of_phy_connect().

Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>