powerpc: Disable 64K hugetlb support when doing 64K SPU mappings

The 64K SPU local store mapping feature is incompatible with the
64K huge pages support due to the inability of some parts of
the memory management to differenciate between them while they
use a different page table format.

For now, disable 64K huge pages when CONFIG_SPU_FS_64K_LS,
in the long run, this can be fixed by making this feature use
the hugetlb page table format.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>