See https://wiki.gentoo.org/wiki/Custom_Initramfs

In order to use uInitramfs (an initramfs with an U-Boot header) you should add the line

#define CONFIG_INITRD_TAG   1

to the U-Boot source file u-boot/src/include/configs/sn9866x.h and recompile U-Boot.

Then tell U-boot to load the uInitramfs file and to pass it as an ATAG enrty to the kernel:

  fatload mmc 0:1 0x01000000 uImage
  fatload mmc 0:1 0x01A00000 uInitramfs
  bootm 0x01000000 0x01A00000

Or, alternatively, you could use initramfs.cpio.gz without an U-Boot wrapper and pass a boot argument initrd=<address_hex>,<size_in_bytes_decimal> to the kernel cmdline:

initrd=0x01A00000,533651

and add a command for loading initramfs image to the u-boot bootcmd:

fatload mmc 0:1 0x01A00000 initramfs.cpio.gz

In both cases the 'root=' kernel cmdline argument is not used by the kernel but you could use it by yourself in the /init and /linuxrc scripts in order to extract a name of appropriate flash partition.

See https://www.kernel.org/doc/html/latest/admin-guide/initrd.html
and https://www.kernel.org/doc/Documentation/filesystems/ramfs-rootfs-initramfs.txt
for more information.