[POWERPC] 4xx: make UIC use generic level irq handler

This patch makes PowerPC 4xx UIC use generic level irq handler instead
of a custom handle_uic_irq() function. We ack only edge irqs in mask_ack
callback, since acking a level irq on UIC has no effect if the interrupt
is still asserted by the device, even if the interrupt is already masked.
So, to really de-assert the interrupt we need to de-assert the external
source first *and* ack it on UIC then. The handle_level_irq() function
masks and ack's the interrupt with mask_ack callback prior to calling
the actual ISR and unmasks it at the end. So, to use it with UIC interrupts
we need to ack level irqs in the unmask callback instead, after the ISR
has de-asserted the external interrupt source. Even if we ack the interrupt
that we didn't handle (unmask/ack it at the end of the handler, while
next irq is already pending) it will not de-assert the irq, untill we
de-assert its exteral source.

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