[POWERPC] Move xics_setup_8259_cascade() into platforms/pseries/setup.c

The code in xics.c to setup the i8259 cascaded irq handler is not really
xics specific, so move it into setup.c - we will clean this up further in
a subsequent patch.

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>