[C72][ENVIRONMENT] Enable mmcautodetect and fdtautodetect function

As for all other boards, the mmc device ID detection has been enabled.
In this way, the source for kernel/dtb and rootfs is automatically set
at runtime with the boot device. So, if the board boots from eMMC, all
other components of the BSP will be picked up from the eMMC; if the boot
is performed from uSD, the source device for the BSP will be the uSD.

Environment variables involved:
kernel_device_id
fdt_device_id
ramfs_device_id
rootfs_device_id

include/configs/seco_imx8mn_c72.h

@@ -224,8 +224,8 @@
 */
 
 #define ENV_MMCAUTODETECT                        "no"
-#define ENV_FDTAUTODETECT                        "no"
-#define ENV_MEMAUTODETECT                        "no"
+#define ENV_FDTAUTODETECT                        "yes"
+#define ENV_MEMAUTODETECT                        "yes"
 
 
 #define SCFG_KERNEL_LOADADDR                               CONFIG_LOADADDR