powerpc/numa: Remove redundant find_cpu_node()

Use of_get_cpu_node, which is a superset of numa.c's find_cpu_node in
a less restrictive section (text vs cpuinit).

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>