powerpc/pseries: Fix build error when NUMA=n

As reported by lkp, if NUMA=n we see a build error:

   arch/powerpc/platforms/pseries/hotplug-cpu.c: In function 'pseries_cpu_hotplug_init':
   arch/powerpc/platforms/pseries/hotplug-cpu.c:1022:8: error: 'node_to_cpumask_map' undeclared
    1022 |        node_to_cpumask_map[node]);

Use cpumask_of_node() which has an empty stub for NUMA=n, and when
NUMA=y does a lookup from node_to_cpumask_map[].

Fixes: bd1dd4c5

 ("powerpc/pseries: Prevent free CPU ids being reused on another node")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210816041032.2839343-1-mpe@ellerman.id.au