dt-bindings: interrupt-controller: Add nuvoton, wpcm450-aic

The WPCM450 AIC ("Advanced Interrupt Controller") is the interrupt
controller found in the Nuvoton WPCM450 SoC and other Winbond/Nuvoton
SoCs.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210406120921.2484986-4-j.neuschaefer@gmx.net