dt-bindings: arm: Add DH electronics i.MX8M Plus DHCOM on PDK3

Add DT compatible string for DH electronics i.MX8M Plus DHCOM on PDK3
evaluation board into YAML DT binding document. This setup is a general
purpose SoM evaluation board.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>