dt-bindings: arm: fsl: imx6ul-kontron: Update bindings

This updates the bindings in order to simplify the devicetree
structure and to add names for the boards that follow the latest
convention used by Kontron marketing.

It also gets rid of the N6xxx notation in the compatibles and
file names, as they are not really used anymore and often result
in confusion.

This is a breaking change, but the impact shouldn't be too big
and it makes usage and maintenance easier in the future.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>