Enable cpuidle for i.MX7S/D using generic ARM cpuidle
driver, below 2 idle states enabled:

1. ARM WFI;
2. SoC WAIT mode.
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>