dt-bindings: arm: fsl: add TQMa8MxNL boards

TQMa8MxNL is a SOM family using NXP i.MX8MN[Q,QL,DL,S,SL] CPU
MBa8Mx is an evaluation mainbord for this SOM

The SOM needs a mainboard, therefore we provide two compatibles here:

"tq,imx8mn-<SOM>" for the module and
"tq,imx8mn-<SOM>-<SBC>" for mainboards

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>