This region is only required if certain fast peripherals make use of such
region for DMAing. It was now enabled on all builds, often resulting in:
```
[134/134] Linking C executable zephyr/zephyr.elf
Memory region Used Size Region Size %age Used
FLASH: 24372 B 248 KB 9.60%
RAM: 4448 B 256 KB 1.70%
DMA_RAM21: 0 GB 16 KB 0.00% <~~ Not used!
DMA_RAM3x_APP: 40 B 4 KB 0.98%
IDT_LIST: 0 GB 32 KB 0.00%
```
Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>
