CDC : Add xfer_fifo & DMA support

[HiFiPhile]

Describe the PR 3rd part of #926

• Add xfer_fifo to CDC class like UAC2.
• Export tu_fifo_buffer_info_t for DMA support.

tud_cdc_rx_wanted_cb is WIP, it needs backward write pointer.

[dmberrycom]

Curious to understand if this DMA feature is likely to make it into the main branch ?

[hathach]

It kind of falling off my radar due to other pending works. I will revise this later on when having more time. There is no reason why we wouldn’t want this, just want to make sure the API is right.

[rrrrrrobot]

Expect dma capability in dwc2