xtensa: add SMP support

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
WIP: xtensa: implement CPU hotplug

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
WIP: xtensa: SMP debug on real LX200

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>