powerpc/pseries: Quieten cede latency printk

The cede latency stuff is relatively new and we don't need to complain about
it not working on older firmware.

Signed-off-by: Anton Blanchard <anton@samba.org>
Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>