bcm53xx: switch USB 3.0 PHY DT description to use MDIO bus

bcm53xx: switch USB 3.0 PHY DT description to use MDIO bus

USB 3.0 PHY is attached to the MDIO bus and should be supported
(accessed) as a MDIO device. This wasn't known initially which resulted
in writing driver that was working with MDIO bus (using some magic
values) without knowing it.

This commit updates DT to properly describe MDIO & USB 3.0 PHY and
enables required kernel drivers.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit 8a175ea2198f59795113a3857f6a742a455ad54f)