power: device: provide device_pm_state_str() with CONFIG_PM=n

So far device_pm_state_str() was built only when CONFIG_PM=y (former
CONFIG_SYSTEM_POWER_MANAGEMENT=y). 'device list' shell
command (CONFIG_DEVICE_SHELL=y) is using that function when
CONFIG_PM_DEVICE=y. This resulted in build failures when CONFIG_PM=n, as
linker could not find its implementation.

Build device_pm_state_str() function regardless of CONFIG_PM value, so
device shell module builds successfully in every case.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>