dt-bindings: arm: fsl: Add carriers for toradex,colibri-imx6dl

Add bindings for Aster, Iris and Iris V2 carrier boards our
Colibri iMX6S/DL may be mated with.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>