pm: converge to suspend state for low power modes

The difference between low power and suspend states is a thin blur line
that is is not clear and most drivers have used indistinctly. This patch
converges to the usage of the suspend state for low power, since
contrary to the low power state, it is used by both system and runtime
device PM. The low power state is still kept, but its future is unclear
and needs some discussion.

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