dt-bindings: arm: fsl: Enumerate Debix Model A Board

Add entries to the list of imx8mp boards denoting the Debix Model A
board from Polyhex Technology Co, along with a more generic entry that
can be used where both the Model A and Model B are supported.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Scally <dan.scally@ideasonboard.com>
Reviewed-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>