ARM: 6402/1: Don't send IPI in smp_send_stop if there's only one CPU

No need to send IPI if there's one CPU, especially when booting
systems with CONFIG_SMP_ON_UP that may not even support IPI.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>