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 <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/all/1bd328b5c9c6cfa633b42af87550f4c7358a05c1.1674499048.git.geert+renesas@glider.beSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

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

@@ -84,6 +84,10 @@ patternProperties:
       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: