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).

首先比较 Adafruit_ZeroDMA 库与 libraries/I2S/src/utility/DMA.h，重点关注通道分配、描述符存储、控制器重置以及用户之间的共存。确定 core 中包含的库或 API 是否最适合与独立于硬件的 ArduinoCore-API 配合使用，并将“完成”定义为：形成一份有文档记录的设计，使多个使用 DMA 的库能够在不发生资源冲突的情况下共存。

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