net: axienet: Properly handle PCS/PMA PHY for 1000BaseX mode

Update the axienet driver to properly support the Xilinx PCS/PMA PHY
component which is used for 1000BaseX and SGMII modes, including
properly configuring the auto-negotiation mode of the PHY and reading
the negotiated state from the PHY.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey@xilinx.com>
Link: https://lore.kernel.org/r/20201028171429.1699922-1-robert.hancock@calian.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>