can: add termination resistor documentation

Add documentation for how to use and setup the switchable termination
resistor support for CAN controllers.

Signed-off-by: Daniel Trevitz <dan@sstrev.com>
Link: https://lore.kernel.org/all/3441354.44csPzL39Z@daniel6430


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>