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 <[e-mail]>
Signed-off-by: Sekhar Nori <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Faiz Abbas <[e-mail]>
Signed-off-by: Marc Kleine-Budde <[e-mail]>

Documentation/devicetree/bindings/net/can/m_can.txt

···
 			  Please refer to 2.4.1 Message RAM Configuration in
 			  Bosch M_CAN user manual for details.
 
+Optional Subnode:
+- can-transceiver	: Can-transceiver subnode describing maximum speed
+			  that can be used for CAN/CAN-FD modes. See
+			  Documentation/devicetree/bindings/net/can/can-transceiver.txt
+			  for details.
 Example:
 SoC dtsi:
 m_can1: can@20e8000 {
···
 	pinctrl-names = "default";
 	pinctrl-0 = <&pinctrl_m_can1>;
 	status = "enabled";
+
+	can-transceiver {
+		max-bitrate = <5000000>;
+	};
 };