dts: arm: st: move power-states to soc dts files

CPU idle states are not board specific. This patch moves STM32 idle
states to the core SoC dts files. Board can always tweak some state
parameters (if needed), but the definition belongs to core SoC dts
files, same as e.g.  peripherals.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>