dt-bindings: interrupt-controller: Add binding for few Microsemi interrupt controllers

Add the Device Tree binding documentation for the Microsemi Jaguar2,
Luton and Serval interrupt controller that is part of the ICPU. It is
connected directly to the MIPS core interrupt controller.

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201125103206.136498-3-gregory.clement@bootlin.com