drivers: dma: add emulated dma driver

Add an emulated DMA driver. Emulation drivers are great to have
for each driver API for multiple reasons:

- providing an ideal / model driver for reference
- potential for configurable backend support
- seamless integration with device tree
- multi-instance, etc, for all supported boards
- fast regression testing of app and library code

Since many other drivers and lbraries depend on DMA, this might
help us to increase test coverage.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>