arm64: dts: imx8mn: Enable cpu-idle driver

Enable i.MX8MN cpu-idle using generic ARM cpu-idle driver, 2 states
are supported, details as below:

root@imx8mnevk:~# cat /sys/devices/system/cpu/cpu0/cpuidle/state0/name
WFI
root@imx8mnevk:~# cat /sys/devices/system/cpu/cpu0/cpuidle/state0/usage
3098
root@imx8mnevk:~# cat /sys/devices/system/cpu/cpu0/cpuidle/state1/name
cpu-pd-wait
root@imx8mnevk:~# cat /sys/devices/system/cpu/cpu0/cpuidle/state1/usage
3078

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