[POWERPC] 4xx: UIC add mask_ack callback

This adds uic_mask_ack_irq() callback to PowerPC 4xx uic code
to avoid kernel crash. It is used for edge-triggered interrupts
by handle_uic_irq().

Signed-off-by: Valentine Barshak <vbarshak@ru.mvista.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>