There are many registers in DM3xx to configure a lot of features; GPIO, SPI, DMA, ...
Almost all the registers are memory mapped registers.
It means you can read/write registers after knowing the physical address of the registers.
Linux is using virtual address mechanism and typically it is not allowed to use physical address in user space.
So you need to do kernel device driver program to use the physical addresses and it is offical way to do.
But it is painful to do small experiments with changing register values.
If you want to do quick and dirty test, here is a useful program.
You can access physical memory in user space, i.e. you can change register values in user space quickly.
Please, check it out