dt-bindings: can: flexcan: list supported processors

The compatible is a pattern match. Explicitly list all possible values.
Also mention that the ls1028ar1 must be followed by lx2160ar1.
Signed-off-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/r/20201001091131.30514-2-michael@walle.ccSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

Documentation/devicetree/bindings/net/can/fsl-flexcan.txt

@@ -4,6 +4,12 @@ Required properties:
 
 - compatible : Should be "fsl,<processor>-flexcan"
 
+  where <processor> is imx8qm, imx6q, imx28, imx53, imx35, imx25, p1010,
+  vf610, ls1021ar2, lx2160ar1, ls1028ar1.
+
+  The ls1028ar1 must be followed by lx2160ar1, e.g.
+   - "fsl,ls1028ar1-flexcan", "fsl,lx2160ar1-flexcan"
+
   An implementation should also claim any of the following compatibles
   that it is fully backwards compatible with: