pm: stop handling devices on PM_STATE_RUNTIME_IDLE

According to the state documentation, this state does not need to handle
devices:

> Runtime idle is a system sleep state in which all of the cores enter
deepest possible idle state and wait for interrupts, no requirements for
the devices, leaving them at the states where they are.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>