Now that percpu allows arbitrary embedding of the first chunk,
powerpc64 can easily be converted to dynamic percpu allocator.
Convert it.  powerpc supports several large page sizes.  Cap atom_size
at 1M.  There isn't much to gain by going above that anyway.
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>