drivers: dma: siwx91x: Use DT to declare descriptors

Silabs siwx91x hardware use specific memory areas to store descriptors
for DMA requests. These areas are tightly coupled between the CPU and
the hardware. This helps in reducing the wait cycles.

Until now these addresses was also hard coded in the DT and in the
linker script. This patch leverage the zephyr,memory-region driver to
centralize the information in the DT.

Then, with this new implementation, the memory mapping is easier to
understand for the reader.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>