Pattern triggering and DAC triggering

[adisuciu]

Pattern triggering consists in activating the DMA when a condition occurs. A usecase for this would be an SPI pattern being generated only when a pin transition occurs (chip select is asserted). Do you have any other usecases for this feature ? Having more usecases can give us a hint on how to implement this. I think we should also take https://github.com/analogdevicesinc/scopy/issues/49 when implementing.