net: axienet: Support dynamic switching between 1000BaseX and SGMII

Newer versions of the Xilinx AXI Ethernet core (specifically version 7.2 or
later) allow the core to be configured with a PHY interface mode of "Both",
allowing either 1000BaseX or SGMII modes to be selected at runtime. Add
support for this in the driver to allow better support for applications
which can use both fiber and copper SFP modules.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Signed-off-by: David S. Miller <davem@davemloft.net>