Hi,
I don't see any DTS attached, you have to change it because they are UART3 flow control.
Which are used in EVK as UART flow control signals mapped to J1003.

In dtsi, they are used as UART 3 pins.
pinctrl_uart3: uart3grp {
fsl,pins = <
MX8MM_IOMUXC_ECSPI1_SCLK_UART3_DCE_RX 0x140
MX8MM_IOMUXC_ECSPI1_MOSI_UART3_DCE_TX 0x140
MX8MM_IOMUXC_ECSPI1_SS0_UART3_DCE_RTS_B 0x140
MX8MM_IOMUXC_ECSPI1_MISO_UART3_DCE_CTS_B 0x140
>;
};
&uart3 {
pinctrl-names = "default";
pinctrl-0 = <&pinctrl_uart3>;
assigned-clocks = <&clk IMX8MM_CLK_UART3>;
assigned-clock-parents = <&clk IMX8MM_SYS_PLL1_80M>;
fsl,uart-has-rtscts;
status = "okay";
};
If you made the changes properly, you should be able to set and clear the GPIO signals with these commands:
#command to turn off the led
root@imx8mnevk:~# echo 0 >> /sys/class/leds/yellow\:status/brightness
#command to turn on the led
root@imx8mnevk:~# echo 1 >> /sys/class/leds/yellow\:status/brightness
Regards,