Use CPU_METHOD_OF_DECLARE() instead. And declare smp method in dts file.

Changelog:
v6:
  * Use hisilicon,hi3620-smp as enable-method property in Hi3620 dts.
Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>
Acked-by: Wei Xu <xuwei5@hisilicon.com>
Signed-off-by: Olof Johansson <olof@lixom.net>