irqchip: Add support for IDT 79rc3243x interrupt controller

IDT 79rc3243x SoCs have rather simple interrupt controllers connected
to the MIPS CPU interrupt lines. Each of them has room for up to
32 interrupts.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210422145330.73452-1-tsbogend@alpha.franken.de