powerpc/8xx: Only 8M pages are hugepte pages now

512k pages are now standard pages, so only 8M pages
are hugepte.

No more handling of normal page tables through hugepd allocation
and freeing, and hugepte helpers can also be simplified.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/2c6135d57fb76eebf70673fbac3dc9e740767879.1589866984.git.christophe.leroy@csgroup.eu