net: dsa: mt7530: support OF-based registration of switch MDIO bus

Currently the MDIO bus of the switches the MT7530 DSA subdriver controls
can only be registered as non-OF-based. Bring support for registering the
bus OF-based.

The subdrivers that control switches [with MDIO bus] probed on OF must
follow this logic to support all cases properly:

No switch MDIO bus defined: Populate ds->user_mii_bus, register the MDIO
bus, set the interrupts for PHYs if "interrupt-controller" is defined at
the switch node. This case should only be covered for the switches which
their dt-bindings documentation didn't document the MDIO bus from the
start. This is to keep supporting the device trees that do not describe the
MDIO bus on the device tree but the MDIO bus is being used nonetheless.

Switch MDIO bus defined: Don't populate ds->user_mii_bus, register the MDIO
bus, set the interrupts for PHYs if ["interrupt-controller" is defined at
the switch node and "interrupts" is defined at the PHY nodes under the
switch MDIO bus node].

Switch MDIO bus defined but explicitly disabled: If the device tree says
status = "disabled" for the MDIO bus, we shouldn't need an MDIO bus at all.
Instead, just exit as early as possible and do not call any MDIO API.

The use of ds->user_mii_bus is inappropriate when the MDIO bus of the
switch is described on the device tree [1], which is why we don't populate
ds->user_mii_bus in that case.

Link: https://lore.kernel.org/netdev/20231213120656.x46fyad6ls7sqyzv@skbuf/

 [1]
Suggested-by: David Bauer <mail@david-bauer.net>
Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20240122053431.7751-1-arinc.unal@arinc9.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>