hw/net/cadence_gem: Support two Ethernet interfaces connected to single MDIO bus

hw/net/cadence_gem: Support two Ethernet interfaces connected to single MDIO bus

The Microchip PolarFire SoC Icicle Kit supports two Ethernet interfaces.
The PHY on each may be connected to separate MDIO busses, or both may be
connected on the same MDIO bus using different PHY addresses.

To be able to support two PHY instances on a single MDIO bus, two properties
are needed: First, there needs to be a flag indicating if the MDIO bus on
a given Ethernet interface is connected. If not, attempts to read from this
bus must always return 0xffff. Implement this property as phy-connected.
Second, if the MDIO bus on an interface is active, it needs a link to the
consumer interface to be able to provide PHY access for it. Implement this
property as phy-consumer.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20251004200049.871646-2-linux@roeck-us.net>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>