powerpc: Use common threadinfo allocator

The core now has a threadinfo allocator which uses a kmemcache when
THREAD_SIZE < PAGE_SIZE.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Link: http://lkml.kernel.org/r/20120505150142.059161130@linutronix.de