irqchip: Add NXP INTMUX interrupt multiplexer support

The Interrupt Multiplexer (INTMUX) expands the number of peripherals
that can interrupt the core:
* The INTMUX has 8 channels that are assigned to 8 NVIC interrupt slots.
* Each INTMUX channel can receive up to 32 interrupt sources and has 1
  interrupt output.
* The INTMUX routes the interrupt sources to the interrupt outputs.

Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200117060653.27485-3-qiangqing.zhang@nxp.com