powerpc/book3s/32: Use MMU_FTR_HPTE_TABLE in head_32.S

Instead of manually patching a blr at hash_page() entry in
MMU_init_hw(), this patch adds a features section in head_32.S

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