ARM: OMAP4+: Allow kexec on SMP variants

Kexec needs omap4_cpu_kill, otherwise kexec will produce on SMP:

kexec_load failed: Invalid argument

Acked-by: Santosh Shilimkar <ssantosh@kernel.org>
Tested-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>