powerpc/numa: Enable USE_PERCPU_NUMA_NODE_ID

Based off 3bccd996 for ia64, convert powerpc to use the generic per-CPU
topology tracking, specifically:

    initialize per cpu numa_node entry in start_secondary
    remove the powerpc cpu_to_node()
    define CONFIG_USE_PERCPU_NUMA_NODE_ID if NUMA

Signed-off-by: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>