[POWERPC] cell: add spu_64k_pages_available() check

Add a function spu_64k_pages_available(), so that we can abstract the
explicity use of mmu_psize_defs() in lssca_alloc.c

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>