can: add dummy_can driver

can: add dummy_can driver

During the development of CAN XL, we found the need of creating a
dummy CAN XL driver in order to test the new netlink interface. While
this code was initially intended to be some throwaway, it received
some positive feedback.

Add the dummy_can driver. This driver acts similarly to the vcan
interface in the sense that it will echo back any packet it receives.
The difference is that it exposes a set on bittiming parameters as a
real device would and thus must be configured as if it was a real
physical interface.

The driver comes with a debug mode. If debug message are enabled (for
example by enabling CONFIG_CAN_DEBUG_DEVICES), it will print in the
kernel log all the bittiming values, similar to what a:

  ip --details link show can0

would do.

This driver is mostly intended for debugging and testing, but some
developers also may want to look at it as a simple reference
implementation.

Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Link: https://patch.msgid.link/20251126-canxl-v8-15-e7e3eb74f889@pengutronix.de
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>