can: ti_hecc: Add TI HECC DT binding documentation

DT binding documentation for TI High End CAN Controller

Signed-off-by: Anton Glukhov <anton.a.glukhov@gmail.com>
Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>