MIPS: lantiq: irq: register the interrupt controllers with irqchip_init

Add support for more interrupt controllers by switching from
of_irq_init() to irqchip_init() in Lantiq's arch_init_irq(). This
requires switching the ICU interrupt controller to use
IRQCHIP_DECLARE(), like a real irqchip driver would do.

This is needed for future changes when new irqchip drivers are
implemented:
- a dedicated driver for the EIU interrupt controller
- a driver for the MSI PIC (Programmable Interrupt Controller) found on
  VRX200 and newer SoCs
- ..or any other driver which uses IRQCHIP_DECLARE

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>