ARM: mvebu: add Device Tree description of the xHCI controller on Armada 375

The Marvell Armada 375 SoCs contain a xHCI controller. This commit
adds the Device Tree description of this interfaces at the SoC level,
and also enables the USB3 port on the Armada 375 DB platform.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Link: https://lkml.kernel.org/r/1400149062-32661-17-git-send-email-gregory.clement@free-electrons.com
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>