dts: bindings: dma: add bindings for an emulated dma controller

Many driver APIs are opting to provide an `emul` driver
implementation that can be used for a number of purposes.

- providing an ideal / model driver for reference
- configurable backends
- seamless integration with device tree
- support for native posix, qemu, and any other board
- fast regression testing of app and library code

Provide an initial set of bindings for `zephyr,dma-emul` devices.

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