There are several options. Common to all of them is that you need to
have some (cross-)compiler to create ARM binary code.
1) there may be drivers in the OS so that you can write a User-
Application using system calls lile open() and ioctl()
2) you may write your own drivers that run within an OS (e.g. Linux)
3) you may modify U-Boot to have additional commands that interact
with your devices
4) you may write your own (mini) OS that is booted through U-Boot (in
a similar way as Linux is booted)
5) I may have forgotten something.
-- hns