powerpc/mm: Fix definitions of FORCE_MAX_ZONEORDER in Kconfig

The current definitions set ranges and defaults for 32 and 64-bit
only using "PPC_STD_MMU" which means hash based MMU. This uselessly
restrict the usefulness for the upcoming 64-bit BookE port, but more
than that, it's broken on 32-bit since the only 32-bit platform
supporting multiple page sizes currently is 44x which does -not-
have PPC_STD_MMU_32 set.

This fixes it by using PPC64 and PPC32 instead.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>