dt-bindings: can: m_can: Document new can transceiver binding

Add information regarding can-transceiver binding. This is especially
important for MCAN since the IP allows CAN FD mode to run significantly
faster than what most transceivers are capable of.

Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>