powerpc: Do not call prink when CONFIG_PRINTK is not defined

When printk() is disabled (CONFIG_PRINTK) at menu item
 General setup
 -> Configure standard kernel features (for small systems)
    -> Enable support for printk
then there should be no printk() calls at all.

Signed-off-by: Márton Németh <nm127@freemail.hu>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>