dt-bindings: arm: Add Data Modul i.MX8M Mini eDM SBC

Add DT compatible string for Data Modul i.MX8M Mini eDM SBC board
into YAML DT binding document. This system is an evaluation board
for various custom display units.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@denx.de>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: NXP Linux Team <linux-imx@nxp.com>
Cc: devicetree@vger.kernel.org
To: linux-arm-kernel@lists.infradead.org
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>