MLK-11357-2 ARM: imx: add cpuidle support for imx7d

Enable cpuidle for i.MX7D, total 3 level idle supported:

1. ARM WFI;
2. WAIT mode;
3. Low power idle with ARM/SCU platform power off.

Only when system in low bus freq mode, system is able to
enter low power idle, and only when both of 2 cores are
in low power idle, ARM/SCU platform will be powered off.

DDR will be put into low power mode when low power idle
is entered.

Signed-off-by: Anson Huang <b20788@freescale.com>