dma: add channels support

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: Tom Rini <trini@konsulko.com>
Signed-off-by: Álvaro 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: Grygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/dma/Kconfig b/drivers/dma/Kconfig
index 4ee6afa..b9b85c6 100644
--- a/drivers/dma/Kconfig
+++ b/drivers/dma/Kconfig
@@ -12,6 +12,13 @@
 	  buses that is used to transfer data to and from memory.
 	  The uclass interface is defined in include/dma.h.
 
+config DMA_CHANNELS
+	bool "Enable DMA channels support"
+	depends on DMA
+	help
+	  Enable channels support for DMA. Some DMA controllers have multiple
+	  channels which can either transfer data to/from different devices.
+
 config TI_EDMA3
 	bool "TI EDMA3 driver"
 	help