MIPS: alchemy: cpu_all_mask is forbidden for clock event devices

change alchemy clock event device cpu_all_mask to cpu_possible_mask.
Gets rid of a warning, which then does the same substitution:
WARNING: CPU: 0 PID: 0 at kernel/time/clockevents.c:468 clockevents_register_device+0x130/0x140
rtcmatch2 cpumask == cpu_all_mask, using cpu_possible_mask instead
Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Linux-MIPS <linux-mips@vger.kernel.org>

arch/mips/alchemy/common/time.c

@@ -81,7 +81,7 @@ static struct clock_event_device au1x_rtcmatch2_clockdev = {
 	.features	= CLOCK_EVT_FEAT_ONESHOT,
 	.rating		= 1500,
 	.set_next_event = au1x_rtcmatch2_set_next_event,
-	.cpumask	= cpu_all_mask,
+	.cpumask	= cpu_possible_mask,
 };
 
 static struct irqaction au1x_rtcmatch2_irqaction = {