[POWERPC] 4xx: rework UIC cascade irq handling

This is a UIC cascade handler rework to use set_irq_chained_handler() for
cascade, just like othe ppc platforms do. With current implementation we have
additional redirection for irq handler and we call generic_handle_irq twice
(once for the primary uic and the other time for handling cascade interrupt).
This causes Ingo's realtime support patch to stop working on 4xx.

Not sure of any other possible problems though, but with
set_irq_chained_handler() we can abolish "struct irqaction cascade" from the
chip descriptor and call generic_handle_irq() once, directly for cascade irq.

Signed-off-by: Valentine Barshak <vbarshak@ru.mvista.com>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>