PM / Runtime: Add no_callbacks flag

Some devices, such as USB interfaces, cannot be power-managed
independently of their parents, i.e., they cannot be put in low power
while the parent remains at full power.  This patch (as1425) creates a
new "no_callbacks" flag, which tells the PM core not to invoke the
runtime-PM callback routines for the such devices but instead to
assume that the callbacks always succeed.  In addition, the
non-debugging runtime-PM sysfs attributes for the devices are removed,
since they are pretty much meaningless.

The advantage of this scheme comes not so much from avoiding the
callbacks themselves, but rather from the fact that without the need
for a process context in which to run the callbacks, more work can be
done in interrupt context.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>