ARM: ixp4xx: Convert to MULTI_IRQ_HANDLER

This rewrites the IXP4xx to use MULTI_IRQ_HANDLER and
create an irqdomain for the irqchip in the platform. We
convert the timer to request the interrupt like any other
driver in the process.

We bump all IRQs to 16+offset to avoid using IRQ 0 and
set NR_IRQS to 512 (the default for most systems).
This conveniently fits with the first 16 IRQs being
pre-allocated when using SPARSE_IRQ.

This is a prerequisite for SPARSE_IRQ and DT boot.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>