powerpc: Refactor update_mmu_cache_range()

On nohash, this function voids except for E500 with hugepages.

On book3s, this function is for hash MMUs only.

Combine those tests and rename E500 update_mmu_cache_range()
as __update_mmu_cache() which gets called by
update_mmu_cache_range().

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/b029842cb6783cbeb43d202e69a90341d65295a4.1695659959.git.christophe.leroy@csgroup.eu