PM / Runtime: Lenient generic runtime pm callbacks

Allow drivers, that belong to subsystems which use the generic
runtime pm callbacks, not to define runtime pm suspend/resume handlers,
by implicitly assuming success in such cases.

This is needed to eliminate nop handlers that would otherwise be
necessary by drivers which enable runtime pm, but don't need
to do anything when their devices are runtime-suspended/resumed.

Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Acked-by: Kevin Hilman <khilman@deeprootsystems.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>