Add driver async API

Add an asynchronous API to the driver. The current synchronous API
is now using the new async API internally. The main new functions
are ethosu_invoke_async() and ethosu_wait(). Every successfull call
to ethosu_invoke_async() must be followed by a call to ethosu_wait()
to get the status of the inference. The wait function can be called
in a blocking or non-blocking mode by specifying the `block`
argument to true/false. The regular synchronous invoke function is
implemented as a invoke_async followed by a wait(block=true) call.

Short sommary of changes:
 - Add an internal ethosu_job struct to keep track of
   inference data and job state.
 - Use async API in blocking mode for normal flow
 - Change default semaphore implementation to binary type
 - Move error prints out of interrupt context
 - Move ethosu_inference_begin() callback to right before HW invoke
 - Always call ethosu_inference_end() callback, even in case of errors
 - On NPU error, do not keep NPU powered after being reset

Change-Id: If4c3c46e3c6732a669e17251bd848dea5765a490