dt-bindings/interrupt-controller: Add DT binding for the Marvell ICU

This commit adds the Device Tree binding documentation for the Marvell
ICU interrupt controller, which collects wired interrupts from the
devices located into the CP110 hardware block of Marvell Armada 7K/8K,
and converts them into SPI interrupts in the GIC located in the AP
hardware block, using the GICP extension.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>