power: Eliminate SYS_PM_* power states.

The power management framework used two different abstractions
to describe power states. The SYS_PM_* given coarse information
what kind of power state (low power or deep sleep) was used,
while the SYS_POWER_STATE_* abstraction provided information
about particular power mode.

This commit removes the SYS_PM_* abstraction as the same
information is already carried in SYS_POWER_STATE_*.

Signed-off-by: Piotr Zięcik <piotr.ziecik@nordicsemi.no>