powerpc/mm: Refactor pte_update() on book3s/32

When CONFIG_PTE_64BIT is set, pte_update() operates on
'unsigned long long'
When CONFIG_PTE_64BIT is not set, pte_update() operates on
'unsigned long'

In asm/page.h, we have pte_basic_t which is 'unsigned long long'
when CONFIG_PTE_64BIT is set and 'unsigned long' otherwise.

Refactor pte_update() using pte_basic_t.

While we are at it, drop the comment on 44x which is not applicable
to book3s version of pte_update().

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/c78912bc8613fb249c3d80aeb1062796b5c49400.1589866984.git.christophe.leroy@csgroup.eu