[POWERPC] Use pseries_setup_i8259_cascade() in pseries_mpic_init_IRQ()

pseries_mpic_init_IRQ() implements the same logic as the xics code did to
find the i8259 cascade irq.  Now that we've pulled that logic out into
pseries_setup_i8259_cascade() we can use it in the mpic code.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>