Add a spidev node inside the spi6 node like below in arch/arm64/boot/dts/ti/k3-j7200-common-proc-board.dts:

+&main_spi6 {
+ pinctrl-names = "default";
+ pinctrl-0 = <&spi6_pins_default>;
+ status="okay";
+
+ spidev@0 {
+
+ spi-max-frequency = <24000000>;
+ reg = <0>;
+ compatible = "linux,spidev";
+};
+};

Enable CONFIG_SPI_SPIDEV=y explicitly in arch/arm64/configs/tisdk_j7-evm_defconfig.

Once you boot Linux one should entries like below:

ls -l /sys/class/spi*
/sys/class/spi_master: total 0 lrwxrwxrwx 1 root root 0 Jun 17 14:17 spi6 -> ../../devices/platform/interconnect@100000/2160000.spi/spi_master/spi6
/sys/class/spidev: total 0 lrwxrwxrwx 1 root root 0 Jun 17 14:17 spidev6.0 -> ../../devices/platform/interconnect@100000/2160000.spi/spi_master/spi6/spi6.0/spidev/spidev6.0