arm64: dts: imx8ulp: add cpuidle node

Add cpuidle node and enable cpuidle for dual cores. The HW mode in
Arm Trusted Firmware is SoC Application Power Domain Sleep mode.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>