[POWERPC] Turn xics_setup_8259_cascade() into a generic pseries_setup_i8259_cascade()

Remove the xics references from xics_setup_8259_cascade(), and merge the
good bits from the almost identical logic in pseries_mpic_init_IRQ().

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>