boards: silabs: siwx91x: Add support for DMAs

Report DMA configuration in the board definitions.

Note the addresses of the DMA buffers are hardcoded in the HAL. So,
these areas have to be declared in the linker file.

Co-authored-by: Sai Santhosh Malae <santhosh.malae@silabs.com>
Signed-off-by: Sai Santhosh Malae <santhosh.malae@silabs.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>