dt-bindings: irqchip: Introduce TISCI Interrupt router bindings

Add the DT binding documentation for Interrupt router driver.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>