config: use ARCH_FORCE_MAX_ORDER=8 on ppc64/ppc64le
    
    Mainline commit 23baf831a32c ("mm, treewide: redefine MAX_ORDER sanely")
    redefined the meaning of MAX_ORDER, and therefore also related
    ARCH_FORCE_MAX_ORDER config option to be one lower than the old value so
    that having ARCH_FORCE_MAX_ORDER=9 with 64KB pages results in build time
    error "Allocator MAX_ORDER exceeds SECTION_SIZE".
    
    Update the values on ppc64 and ppc64le architectures from 9 to 8 to
    preserve the old behaviour and fix the build error.