tests: adc: add configuration and overlay for DMA support on Silabs IADC

This commit introduces a new configuration file for DMA support specific
to the Silabs IADC, along with updates to the test case YAML and device
tree overlay for the xg29_rb4412a board. The changes ensure that the ADC
driver can utilize DMA operations effectively.

Signed-off-by: Martin Hoff <martin.hoff@silabs.com>
9 files changed