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

This commit adds the Device Tree binding documentation for the Marvell
GICP, an extension to the GIC that allows to trigger GIC SPI interrupts
using memory transactions. It is used by the ICU unit in the Marvell
CP110 block to turn wired interrupts inside the CP into SPI interrupts
at the GIC level in the AP.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>