powerpc: Export CMO_PageSize

This fixes an error building powerpc allmodconfig:

ERROR: "CMO_PageSize" [arch/powerpc/platforms/pseries/cmm.ko] undefined!

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>