powerpc/ps3: Add __init to ps3_smp_probe

Add an __init annotation to the ps3_smp_probe() routine.
Fixes build warnings like these when
CONFIG_DEBUG_SECTION_MISMATCH=y:

 WARNING: Section mismatch in reference from the function .ps3_smp_probe()

Signed-off-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>