dt-bindings: arm: Document Armadeus SoM and Dev boards devicetree binding

Document the following Armadeus SoM and Dev boards devicetree binding
already supported:
- armadeus,imx27-apf27 and armadeus,imx27-apf27dev
- armadeus,imx28-apf28 and armadeus,imx28-apf28dev
- armadeus,imx51-apf51 and armadeus,imx51-apf51dev
- armadeus,imx6{q,dl}-apf6 and armadeus,imx{q,dl}-apf6dev
- armadeus,imx6{ul,ull}-opos6ul and armadeus,imx{ul,ull}-opos6uldev

Signed-off-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>