net: phy: Add gmiitorgmii converter support

This patch adds support for gmiitorgmii converter.

The GMII to RGMII IP core provides the Reduced Gigabit Media
Independent Interface (RGMII) between Ethernet physical media
Devices and the Gigabit Ethernet controller. This core can
Switch dynamically between the three different speed modes of
Operation by configuring the converter register through mdio write.

MDIO interface is used to set operating speed of Ethernet MAC.

This converter sits between the MAC and the external phy
MAC <==> GMII2RGMII <==> RGMII_PHY

Signed-off-by: Kedareswara rao Appana <appanad@xilinx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>