can: sja1000: add documentation for Technologic Systems version

This commit adds documentation for the Technologic Systems version of
SJA1000. The difference with the NXP version is in the way the registers
are accessed.

Signed-off-by: Damien Riegel <damien.riegel@savoirfairelinux.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>