dt-bindings: arm: imx: Add the soc binding for imx8mm

Add the soc & board binding for i.MX8MM.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>