dt-bindings: arm: Add Polyhex DEBIX SOM A based boards

Add devicetree bindings for i.MX8MP based DEBIX SOM A and SOM A I/O
baseboard:
  - https://debix.io/hardware/debix-som-a.html
  - https://debix.io/hardware/debix-som-a-io-board.html



Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>