dt-bindings: arm: fsl: add TQ-Systems boards based on i.MX6Q/QP/DL

dt-bindings: arm: fsl: add TQ-Systems boards based on i.MX6Q/QP/DL

The TQ-Systems MBa6x mainboard can be used with TQMa6 variants with
i.MX6Q/QP/DL SoCs (TQMa6Q/QP/DL). The TQMa6Q and DL exist in two variants:
The newer variants "A" have a hardware workaround for Erratum ERR006687,
while variants "B" are missing such a workaround, so it needs to be
handled in software. The erratum was fixed in i.MX6QP, so no "A" variant
of the TQMa6QP exists.

Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>