ARM: keystone: Enable SMP support on Keystone machines

Add basic SMP support for Keystone machines. This does not
include support for CPU hotplug for now.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: arm@kernel.org

Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>