powerpc/xive: Export XIVE IPI information for online-only processors.

Cédric pointed out that XIVE IPI information exported via sysfs
(debug/powerpc/xive) display empty lines for processors which are
not online.

Switch to using for_each_online_cpu() so that information is
displayed for online-only processors.

Reported-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Sachin Sant <sachinp@linux.ibm.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/164146703333.19039.10920919226094771665.sendpatchset@MacBook-Pro.local