ADC fifo DMA

[nilclass]

DMA support for free-running ADC capture.

The basic API looks like this:

// without DMA:
let adc_fifo = adc.build_fifo()
    .something_something()
    .start();

do_something_with_fifo(adc_fifo);

// with DMA:
let adc_fifo = adc.build_fifo()
    .something_something()
    .enable_dma()
    .prepare();

let transfer = dma::single_buffer::Config::new(dma.ch0, adc_fifo.dma_read_target(), some_write_target).start();
adc_fifo.resume();

do_something_else_while_dma_does_all_the_work();

To enable this functionality, the following methods were added:

• AdcFifoBuilder::enable_dma: sets FCS.DREQ_EN = 1 and FCS.THRESH = 1
• AdcFifoBuilder::prepare: same as start, but does not set CS.START_MANY just yet
• AdcFifo::resume / AdcFifo::pause: control if capturing is active (CS.START_MANY) without reconfiguring the FIFO in any other way
• AdcFifo::is_paused: can be used to check the current state
• AdcFifo::clear: reads the FIFO until empty, discarding values
• AdcFifo::dma_read_target: returns a struct that implements dma::ReadTarget, and can be used to construct the transfer. (If dma::ReadTarget were implemented on AdcFifo directly, it wouldn't be possible to resume() the FIFO after starting the DMA transfer, because the transfer would take ownership of the AdcFifo)

[jannic]

I finally found the time to actually try it out on hardware.
LGTM, and thanks for contributing!