net: txgbe: Add software nodes to support phylink

Register software nodes for GPIO, I2C, SFP and PHYLINK. Define the
device properties.

Signed-off-by: Jiawen Wu <jiawenwu@trustnetic.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Piotr Raczynski <piotr.raczynski@intel.com>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>