powerpc/mm: Fix 40x and 8xx vs. _PAGE_SPECIAL

The test to check whether we have _PAGE_SPECIAL defined is broken,
since we always define it, just not always to a meaninful value :-)

That broke 8xx and 40x under some circumstances.

This fixes it by adding _PAGE_SPECIAL for both of these since they
had a free PTE bit, and removing the condition around advertising
it.

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