pm: device_runtime: Allow to use a dedicated wq

Device runtime is using the system workqueue to do operations
that are mostly blockers (suspend a device). This should not happen.

This commit adds an option to use dedicated queue for the device runtime
async operations.

The test for this API was assuming that the system workqueue
priority (which is cooperative) so we need to change the test priority
to be lower than the device runtime workqueue priority.

Signed-off-by: Flavio Ceolin <flavio@hubblenetwork.com>