dt-bindings: interrupt-controller: Add binding for the Microsemi Ocelot interrupt controller

Add the Device Tree binding documentation for the Microsemi Ocelot
interrupt controller that is part of the ICPU. It is connected directly to
the MIPS core interrupt controller.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>