can: holt_hi311x: document device tree bindings

Document the HOLT HI-311x CAN device tree bindings.

Signed-off-by: Akshay Bhat <nodeax@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>