dts: bindings: add binding for NXP FlexCAN CANFD variant

Add a devicetree binding for the CAN-FD capable variant of the NXP FlexCAN
controller. Add example devicetree snippets to both NXP FlexCAN and NXP
FlexCAN-FD binding documentation to limit confusion.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>