pseries: Move XICS initialization before cpu initialization

Currently, the pseries machine initializes the cpus, then the XICS
interrupt controller.  However, to support the upcoming in-kernel XICS
implementation we will need to initialize the irq controller before the
vcpus.  This patch makes the necesssary rearrangement.  This means the
xics init code can no longer auto-detect the number of cpus ("interrupt
servers" in XICS terminology) and so we must pass that in explicitly from
the platform code.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Ben Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>
