PM: sleep: Make suspend of devices more asynchronous

In analogy with previous changes, make device_suspend_late() and
device_suspend_noirq() start the async suspend of the device's parent
after the device itself has been processed and make dpm_suspend_late()
and dpm_noirq_suspend_devices() start processing "async" leaf devices
(that is, devices without children) upfront so they don't need to wait
for the other devices they don't depend on.

This change reduces the total duration of device suspend on some systems
measurably, but not significantly.

Suggested-by: Saravana Kannan <saravanak@google.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/1924195.CQOukoFCf9@rjwysocki.net