dt-bindings: arm: Add DH electronics i.MX8M Plus DHCOM and PDK2

dt-bindings: arm: Add DH electronics i.MX8M Plus DHCOM and PDK2

Add DT compatible string for DH electronics i.MX8M Plus DHCOM and PDK2
into YAML DT binding document. This system is a general purpose SoM and
evaluation board.

Acked-by: Rob Herring <robh@kernel.org>
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
Signed-off-by: Shawn Guo <shawnguo@kernel.org>