ARM: imx: add i.MX7ULP cpuidle support

This patch adds cpuidle support for i.MX7ULP, 3 cpuidle
states supported as below:

1. WFI, just ARM wfi;
2. WAIT mode, mapped to SoC's partial stop mode #3;
3. STOP mode, mapped to SoC's partial stop mode #1.

In WAIT mode, system clock and bus clock will be enabled;
In STOP mode, system clock and bus clock will be disabled.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>