dt-bindings: arm: fsl: fix DEBIX binding

The current imx8mp-debix-model-a.dts uses all three compatibles. Fix the
corresponding bindings by adding an own entry for it. Adapt the comment
for the "polyhex,imx8mp-debix" binding to make it clear that this should
only be used for DEBIX Model A/B i.MX8MP SBCs.

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