This adds channels support for dma controllers that have multiple channels
which can transfer data to/from different devices (enet, usb...).

DMA channle API:
 dma_get_by_index()
 dma_get_by_name()
 dma_request()
 dma_free()
 dma_enable()
 dma_disable()
 dma_prepare_rcv_buf()
 dma_receive()
 dma_send()
Reviewed-by: NTom Rini <trini@konsulko.com>
Signed-off-by: NÁlvaro Fernández Rojas <noltari@gmail.com>
[grygorii.strashko@ti.com: drop unused dma_get_by_index_platdata(),
 add metadata to send/receive ops, add dma_prepare_rcv_buf(),
 minor clean up]
Signed-off-by: NGrygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: NSimon Glass <sjg@chromium.org>