powerpc/powernv: Fix build error in opal-imc.c when NUMA=n

When building a random powerpc kernel I hit this build error:

  arch/powerpc/platforms/powernv/opal-imc.c:130:13: error : assignment
  discards « const » qualifier from pointer target type
  [-Werror=discarded-qualifiers]
     l_cpumask = cpumask_of_node(nid);
             ^

This happens because when CONFIG_NUMA=n cpumask_of_node() returns a
const pointer.

This patch simply adds const to l_cpumask to fix this issue.
Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Reviewed-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
[mpe: Flesh out change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/powernv/opal-imc.c

@@ -123,7 +123,7 @@ static int imc_pmu_create(struct device_node *parent, int pmu_index, int domain)
 static void disable_nest_pmu_counters(void)
 {
 	int nid, cpu;
-	struct cpumask *l_cpumask;
+	const struct cpumask *l_cpumask;
 
 	get_online_cpus();
 	for_each_online_node(nid) {