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).

まず Adafruit_ZeroDMA ライブラリと libraries/I2S/src/utility/DMA.h を比較し、チャネルの割り当て、ディスクリプタの格納、コントローラのリセット、ユーザー間の共存に焦点を当てます。ハードウェア非依存の ArduinoCore-API に最も適しているのが core に含めるライブラリか API かを判断し、複数の DMA 使用ライブラリがリソースの競合なしに共存できることを示す文書化された設計を完成の定義とします。

索引モデルが issue の本文から書いたものです。