Circular DMA driver implementation

[tyler-gilbert]

For a circular DMA setup, reading channel 0 will set the first buffer while reading channel 1 will set the second buffer.

To get the driver started, do an async read on channel 0 then on channel 1. When the channel 0 read finishes, do another read on channel 0. Likewise when channel 1 completes, do another read on channel 1.

For the i2s_ffifo, channel 0 will be read to the first open slot, channel 1 will be read to the seconds spot, then channel 0 will be read to the third spot and so on until the ffifo overflows.

[tyler-gilbert]

New Idea:

• Use the half callback and set MCU_EVENT_FLAG_LOW with MCU_EVENT_FLAG_DATA_READY and use MCU_EVENT_FLAG_HIGH
• A single call to _dma_read() will create a chain of reads that lasts until the read is aborted -- callback returns 0
• A read() on a device like this will abort after reading half the data. This approach must be used with an ffifo or similar structure -- maybe direct support on the switchboard could work