powerpc/mm: Drop CONFIG_PPC_HAS_HASH_64K

The relation between CONFIG_PPC_HAS_HASH_64K and CONFIG_PPC_64K_PAGES is
painfully complicated.

But if we rearrange it enough we can see that PPC_HAS_HASH_64K
essentially depends on PPC_STD_MMU_64 && PPC_64K_PAGES.

We can then notice that PPC_HAS_HASH_64K is used in files that are only
built for PPC_STD_MMU_64, meaning it's equivalent to PPC_64K_PAGES.

So replace all uses and drop it.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>