powerpc/8xx: Only define APG0 and APG1

Since the 8xx implements hardware page table walk assistance,
the PGD entries always point to a 4k aligned page, so the 2 upper
bits of the APG are not clobbered anymore and remain 0. Therefore
only APG0 and APG1 are used and need a definition. We set the
other APG to the lowest permission level.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>