Should provide DMA library

The SAMD core supports DMA, but this cannot be easily used from a library or sketch currently. In particular, multiple users of the DMA module need some coordination about what channels are in use, where the descriptor memory is located (I think), when to reset the DMA controller, etc.

Currently, I think there is the [Adafruit_ZeroDMA](https://github.com/adafruit/Adafruit_ZeroDMA) library that does this, but the [I2S library](https://github.com/arduino/ArduinoCore-samd/blob/master/libraries/I2S/src/utility/DMA.h) also has a DMA coordinator embedded (so these two will not play together nicely).

A single official library contained in the core (or perhaps an API contained in the core itself) would be good here. This could be a fully featured DMA library, but it might also be useful to have a minimal DMA coordinator library (that e.g. only handles resetting/initializing the controller, allocating channels and descriptors and maybe more, leaving all the channel-specific configuration that can coexist to third party or other libraries).

Beginne mit dem Vergleich der Adafruit_ZeroDMA-Bibliothek mit libraries/I2S/src/utility/DMA.h und konzentriere dich dabei auf die Kanalzuweisung, die Speicherung der Deskriptoren, das Zurücksetzen des Controllers und das konfliktfreie Zusammenspiel zwischen Nutzern. Ermittle, ob eine im Core enthaltene Bibliothek oder API am besten zur hardwareunabhängigen ArduinoCore-API passt, und definiere „fertig“ als ein dokumentiertes Design, das mehreren DMA-nutzenden Bibliotheken ermöglicht, ohne Konflikte um Ressourcen zusammenzuarbeiten.

Vom Indexierungsmodell aus dem Issue-Text verfasst.