dt-bindings: arm: fsl: document the LX2160A BlueBox 3 boards

Document the compatible string for the LX2160A system that is part of
the BlueBox 3. Also add a separate compatible string for Rev A, since
technically it uses a different device tree.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>