net: phy: Add gmiitorgmii converter support

This patch adds support for gmiitorgmii converter.
This converter sits between the MAC and the external phy
MAC <==> GMII2RGMII <==> RGMII_PHY.
The ethernet driver probes this bridge and this bridge driver
probes real phy driver and invokes the real phy functionalities
as requested. This bridge just needs to be configured based on
real phy negotiated speed and duplex.

Signed-off-by: Siva Durga Prasad Paladugu <siva.durga.paladugu@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>