Phy: DT binding documentation for the Armada 375 USB cluster binding

Armada 375 comes with an USB2 host and device controller and an USB3
controller. The USB cluster control register allows to manage common
features of both USB controllers. This commit adds the Device Tree
binding documentation for this piece of hardware.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>