dts: arm: st: stm32h5: relocate `power-states` node

Relocate the `power-states` node from under the `soc` node to
the `cpus` node, making it consistent with other STM32 SoC series.

This resolves the device-tree warning:
(simple_bus_reg): /soc/power-states: missing or empty reg/ranges property.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>