[POWERPC] Enable CONFIG_FORCE_MAX_ZONEORDER for all PowerPC, and make selectable

This enables the FORCE_MAX_ZONEORDER Kconfig option for all PowerPC
systems.  Previously, it was enabled only for 64-bit systems.  We also
make the option selectable from the menu, so that the user can specify
different values.  This is useful for 32-bit systems that need to
allocate more than 4MB of physically contiguous memory.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>