dt-bindings: Add description of Socionext EXIU interrupt controller

Add a description of the External Interrupt Unit (EXIU) interrupt
controller as found on the Socionext SynQuacer SoC.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>