The cpu_setup() handler currently takes a 'XICSState *' argument to
grab the kernel ICP file descriptor. This interface can be simplified
by using the 'xics' backlink of the ICP object.

This change is also required by subsequent patches which makes use of
the QOM interface for XICS.
Signed-off-by: NCédric Le Goater <clg@kaod.org>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>