powerpc/mm: Make pte_pgprot return all pte bits

Other archs do the same and instead of adding required pte bits (which
got masked out) in __ioremap_at(), make sure we filter only pfn bits
out.

Fixes: 26973fa5

 ("powerpc/mm: use pte helpers in generic code")
Reviewed-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>