ACPI / PM: Common string representations of device power states

The function returning string representations of ACPI device power
states, state_string((), is now static, because it is only used
internally in drivers/acpi/bus.c.  However, it will be used outside
of that file going forward, so rename it to
acpi_power_state_string(), add a kerneldoc comment to it and add its
header to acpi_bus.h.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>