dt-bindings: can: renesas,rcar-canfd: Add transceiver support

Add support for describing CAN transceivers as PHYs.

While simple CAN transceivers can do without, this is needed for CAN
transceivers like NXP TJR1443 that need a configuration step (like
pulling standby or enable lines), and/or impose a bitrate limit.

Signed-off-by: Geert Uytterhoeven <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Link: https://lore.kernel.org/all/1bd32[e-mail]
Signed-off-by: Marc Kleine-Budde <[e-mail]>

Documentation/devicetree/bindings/net/can/renesas,rcar-canfd.yaml

···
       The controller supports multiple channels and each is represented as a
       child node.  Each channel can be enabled/disabled individually.
 
+    properties:
+      phys:
+        maxItems: 1
+
     additionalProperties: false
 
 required: