drivers: dma: siwx91x: Allow static allocation of DMA channel descriptors

Some instances of DMA (dma0) can use the normal sram to store their
descriptors. In this case, it makes sense to allow the linker to
allocate the memory rather than tweaking the memory layout.

So, if the attribute silabs,sram-region is not defined, use a statically
allocated buffer.

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