PM / sleep: Set pm_generic functions to NULL for !CONFIG_PM_SLEEP

Previously only a subset of the functions were defined and set to NULL
while !CONFIG_PM_SLEEP. Let's make them all available so they can be
used no matter of CONFIG_PM_SLEEP or not.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>