rent patch, I know do:
mpc52xx_irqhost = irq_alloc_host(IRQ_HOST_MAP_LINEAR, NR_IRQS, 
&mpc52xx_irqhost_ops, -1);

Which should be ok.

NR_IRQ, it's a bit violent :) It's also the max number of virtual IRQs
and thus has nothing to do with your HW numbers. Use the max number of
HW IRQs here.

Ben.