dt-bindings: interrupt-controller: Add binding for NXP INTMUX interrupt multiplexer

This patch adds the DT bindings for the NXP INTMUX interrupt multiplexer
for i.MX8 family SoCs.

Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200117060653.27485-2-qiangqing.zhang@nxp.com