xtensa: SMP: mark each possible CPU as present

Otherwise it is impossible to enable CPUs after booting with 'maxcpus'
parameter.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/kernel/smp.c

@@ -83,7 +83,7 @@ void __init smp_prepare_cpus(unsigned int max_cpus)
 {
 	unsigned i;
 
-	for (i = 0; i < max_cpus; ++i)
+	for_each_possible_cpu(i)
 		set_cpu_present(i, true);
 }