net: xilinx_axi: add PCS/PMA PHY

net: xilinx_axi: add PCS/PMA PHY

If we bridge an external PHY to Xilinx's PCS/PMA PHY and would like to
get and set the real status of the PHY facing the external world. Then
we should phy_connect() to the external PHY instead of the PCS/PMA one.
Thus, we add a pcs-handle DT entry, which have been merged in Linux, and
leave the configuration of it to the driver itself.

Unlike Linux, where the PCS/PMA PHY is managed by phylink, managing the
PCS/PMA PHY is only internal to the driver in U-Boot. The PCS/PMA PHY
pressents only when the phy-mode is configured as SGMII or 1000Base-X,
so it is always 1 Gbps and full-duplex and we may skip passing link
information out.

Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
Reviewed-by: Greentime Hu <greentime.hu@sifive.com>
Link: https://lore.kernel.org/r/20221101035800.912644-2-andy.chiu@sifive.com
Signed-off-by: Michal Simek <michal.simek@amd.com>