phy: add support for USB cluster on the Armada 375 SoC

The Armada 375 SoC 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 a driver integrated in the generic PHY framework to
control this USB cluster feature.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
[ kishon@ti.com : Made it to use the updated devm_phy_create API and
		  soem cosmentic changes in Kconfig file.]
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>