powerpc/8xx: Use generic permission masks

8xx already has _PAGE_NA and _PAGE_RO. So add _PAGE_ROX, _PAGE_RW and
_PAGE_RWX and remove specific permission masks.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/5d0b5ce43485f697313eee4326ddff97157fb219.1695659959.git.christophe.leroy@csgroup.eu