ARM: imx: add cpuidle support for i.mx6sx

Add cpuidle support for i.MX6SX, derive from i.MX6Q's
cpuidle, two levels supported:

1. WFI;
2. WAIT mode.

Signed-off-by: Anson Huang <b20788@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>