ATF and Linux support system suspend, so we don't need define
idle-states to support suspend. RK3368 don't support any
idle state other then WFI. Just remove it.
Change-Id: Ifa95862b4998287560cd2214ff6b5763a9d6ce02 Signed-off-by: Huang, Tao <huangtao@rock-chips.com>