arm64: dts: imx8mp: add cpuidle state "cpu-pd-wait"

So far, only WFI is supported on i.MX8mp platform. Add support for
deeper cpuidle state "cpu-pd-wait" that would allow for better power
usage during runtime. This is a port from NXP downstream kernel.

Signed-off-by: Catalin Popescu <catalin.popescu@leica-geosystems.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>