powerpc/8xx: Prepare handlers for _PAGE_HUGE for 512k pages.

Prepare ITLB handler to handle _PAGE_HUGE when CONFIG_HUGETLBFS
is enabled. This means that the L1 entry has to be kept in r11
until L2 entry is read, in order to insert _PAGE_HUGE into it.

Also move pgd_offset helpers before pte_update() as they
will be needed there in next patch.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/21fd1de8fba781bededa9474a5a9374aefb1f849.1589866984.git.christophe.leroy@csgroup.eu