power: policy: remove deep sleep states from residency policy

Deep sleep states are documented to include SoC-level power gating,
i.e. the SoC loses context.  In practice entering a deep sleep state
requires an external wakeup mechanism that may need to restart the
application.

Such states are too dangerous to enter automatically based on an
expected duration of sleep, especially since a "forever" sleep that
would normally be woken as a result of a peripheral event would select
the deepest sleep state available.  Limit the sleep levels selected by
a residency policy to ones in which the CPU may be stopped, but will
not lose execution context.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>