b53: Add BCM53128 switch support
b53: Make b53_switch_init static
b53: mdio: Avoid re-registering the same switch device

When setting the associated interface down and up again a new
switch device will be registered due to b53_phy_config_init
doing the necessary allocations and registrations.

Instead, register the switch device already in b53_phy_probe.
b53: Use phy_drivers_register to register multiple phy drivers at once