bcm53xx: add BCM53573 Ethernet fix sent upstream for v6.6

bcm53xx: add BCM53573 Ethernet fix sent upstream for v6.6

It seems that DSA-based b53 driver never worked with BCM53573 SoCs and
BCM53125.

In case of swconfig-based b53 this fixes a regression. Switching bgmac
from using mdiobus_register() to of_mdiobus_register() resulted in MDIO
device (BCM53125) having of_node set (see of_mdiobus_register_phy()).
That made downstream b53 driver read invalid data from DT and broke
Ethernet support.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit 79fd3e62b4910731c13692b2daa2083e0f95c023)