arm64: dts: imx8mn-evk: Pass GPIO_OPEN_DRAIN flag

scl-gpios and sda-gpios need GPIO_OPEN_DRAIN flag, otherwise the
following run-time warning is seen:

gpio-144 (scl): enforced open drain please flag it properly in DT/ACPI DSDT/board file
gpio-146 (scl): enforced open drain please flag it properly in DT/ACPI DSDT/board file

Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>