PM: sleep: Make async suspend handle suppliers like parents

Avoid starting "async" suspend processing upfront for devices that have
consumers and start "async" suspend processing for a device's suppliers
right after suspending the device itself.

Suggested-by: Saravana Kannan <saravanak@google.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Link: https://patch.msgid.link/3384525.44csPzL39Z@rjwysocki.net