can: ucan: add driver for Theobroma Systems UCAN devices

The UCAN driver supports the microcontroller-based USB/CAN
adapters from Theobroma Systems. There are two form-factors
that run essentially the same firmware:

* Seal: standalone USB stick ( https://www.theobroma-systems.com/seal )

* Mule: integrated on the PCB of various System-on-Modules from
  Theobroma Systems like the A31-µQ7 and the RK3399-Q7
  ( https://www.theobroma-systems.com/rk3399-q7

 )

The USB wire protocol has been designed to be as generic and
hardware-indendent as possible in the hope of being useful for
implementation on other microcontrollers.

Signed-off-by: Martin Elshuber <martin.elshuber@theobroma-systems.com>
Signed-off-by: Jakob Unterwurzacher <jakob.unterwurzacher@theobroma-systems.com>
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Wolfgang Grandegger <wg@grandegger.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>