dt-bindings: arm: Document i.MX8QXP AI_ML board binding

Document devicetree binding of i.MX8QXP AI_ML board from Einfochips.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>