ARM: rockchip: convert smp bringup to CPU_METHOD_OF_DECLARE

With the newly introduced CPU_METHOD_OF_DECLARE is not necessary anymore
to reference the relevant smp_ops in the board file, but instead it can
simply be set by the enable-method property of the cpu nodes.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Olof Johansson <olof@lixom.net>