dt-bindings: arm: Add SolidRun LX2162A SoM & Clearfog Board

SolidRun now have 2 product lines around NXP Layerscape SoC:
- LX2160A COM Express 7
- LX2162A System on Module

LX2162 is a smaller package and reduced feature set to LX2160A;
LX2162 SoM is also a smaller form factor and reduced feature set to CEX.

Since both product lines are physically incompatible,
the existing group "SolidRun LX2160A based Boards" has been renamed to
include "CEX" in its name, meaning products based on LX2160A COM Express
Module, following this pattern:
"solidrun,<board>", "solidrun,lx2160a-cex", "fsl,lx2160a"

Add DT compatible for both SolidRun LX2162A SoM, and LX2162 Clearfog
boards to a new group based on LX2162A SoM, following this pattern:
"solidrun,<board>", "solidrun,lx2162a-som", "fsl,lx2160a"

Signed-off-by: Josua Mayer <josua@solid-run.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>