ppc/xics: An ICS with offset 0 is assumed to be uninitialized

This will make life easier for dealing with dynamically configured
ICSes such as PHB3

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>