dt-bindings: can: flexcan: convert fsl,*flexcan bindings to yaml

In order to automate the verification of DT nodes convert
fsl-flexcan.txt to fsl,flexcan.yaml

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201022075218.11880-3-o.rempel@pengutronix.de
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>