PM: suspend: Fix pm_suspend_target_state handling for !CONFIG_PM

Move the pm_suspend_target_state definition for CONFIG_SUSPEND
unset from the wakeup code into the headers so as to allow it
to still be used elsewhere when CONFIG_SUSPEND is not set.

Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
[ rjw: Changelog and subject edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>