net: macb: rename bp->sgmii_phy field to bp->phy

net: macb: rename bp->sgmii_phy field to bp->phy

The bp->sgmii_phy field is initialised at probe by init_reset_optional()
if bp->phy_interface == PHY_INTERFACE_MODE_SGMII. It gets used by:
 - zynqmp_config: "cdns,zynqmp-gem" or "xlnx,zynqmp-gem" compatibles.
 - mpfs_config: "microchip,mpfs-macb" compatible.
 - versal_config: "xlnx,versal-gem" compatible.

Make name more generic as EyeQ5 requires the PHY in SGMII & RGMII cases.

Drop "for ZynqMP SGMII mode" comment that is already a lie, as it gets
used on Microchip platforms as well. And soon it won't be SGMII-only.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
Link: https://patch.msgid.link/20251023-macb-eyeq5-v3-4-af509422c204@bootlin.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>