dt-bindings: phy: Modify Broadcom NS USB 3.0 PHY binding to use MDIO

Thanks to work done by Broadcom explaining their USB 3.0 PHY details we
know it's attached to the MDIO bus. Use this knowledge to update the
binding: make it a subnode to the MDIO bus and rework way of specifying
required registers. This will describe hardware more precisely and will
allow to support (describe) more devices attached to the MDIO.

While compatibility strings remain the same there isn't a direct
conflict (compatibility breakage) for the binding. Originally it wasn't
supposed to be used for MDIO subnode so this change should be safe
unless some operating system was probing MDIO subnodes as standalone
devices.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>