net: axienet: Support for RGMII

This patch adds support for the RGMII. The h/w configuration
parameter C_PHY_TYPE, which represents the interface configured in
the design, is used to differentiate various interfaces supported
by AXI Ethernet.

Signed-off-by: Srikanth Thokala <sthokal@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>