powerpc/numa: make vphn_enabled, prrn_enabled flags const

Previous changes have made it so these flags are never changed;
enforce this by making them const.
Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200612051238.1007764-6-nathanl@linux.ibm.com

arch/powerpc/mm/numa.c

@@ -1132,8 +1132,8 @@ struct topology_update_data {
 #define TOPOLOGY_DEF_TIMER_SECS	60
 
 static cpumask_t cpu_associativity_changes_mask;
-static int vphn_enabled;
-static int prrn_enabled;
+static const int vphn_enabled;
+static const int prrn_enabled;
 static void reset_topology_timer(void);
 static int topology_timer_secs = 1;
 static int topology_inited;