boards: nordic: do not enable DMA_RAM21 region by default

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>