powerpc: Enforce sane MAX_ORDER

powerpc uses CONFIG_FORCE_MAX_ZONEORDER, and some things depend on it
being at least 10 when 64k pages are not configured (notably the dart
iommu code with CONFIG_PM). The defaults are fine, but when going from a
64K pages config to one without 64K pages, MAX_ORDER stays at 9 which is
too low for 4K pages.

This patch makes the Kconfig enforce at least the defaults.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>