Merge #648 #656

648: Fix nRF Saadc continuous sampling r=Dirbaio a=huntc

Starting the sampling task prior to starting the SAADC peripheral can lead to unexpected buffer behaviour with multiple channels. We now provide an init callback at the point where the SAADC has started for the first time. This callback can be used to kick off sampling via PPI.

We also need to trigger the SAADC to start sampling the next buffer when the previous one is ended so that we do not drop samples - the major benefit of double buffering.

Given these additional tasks, we now simplify the API by passing in the TIMER and two PPI channels.

As a bonus, we provide an async `calibrate` method as it is recommended to use before starting up the sampling.

The example has been updated to illustrate these new features along with the simplified API.

The changes here have been tested on my nRF52840-DK.

656: stm32: Refactor DMA interrupts r=Dirbaio a=GrantM11235

Previously, every dma interrupt handler called the same `on_irq`
function which had to check the state of every dma channel.

Now, each dma interrupt handler only calls an `on_irq` method for its
corresponding channel or channels.

Co-authored-by: huntc <[email protected]>
Co-authored-by: Grant Miller <[email protected]>