dma: add channels support

In order to add bcm6348-enet support, dma-uclass must be extended to support dma channels and reworked to operate like the other dm uclass (clk, reset...).

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A DMA is a feature of computer systems that allows certain hardware subsystems to access main system memory, independent of the CPU. DMA channels are typically generated externally to the HW module consuming them, by an entity this API calls a DMA provider. This API provides a standard means for drivers to enable and disable DMAs, and to copy, send and receive data using DMA.

DMA channel 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()

A driver that implements UCLASS_DMA is a DMA provider. A provider will often implement multiple separate DMAs channels, since the hardware it manages often has this capability. dma_uclass.h describes the interface which DMA providers must implement.

DMA consumers/clients are the HW modules driven by the DMA channels.

DMA consumer DMA_MEM_TO_DEV (transmit) usage example (based on networking). Note. In u-boot dma_send() is sync operation always - it'll start transfer and will poll for it to complete:
- get/request dma channel

	struct dma dma_tx;
	ret = dma_get_by_name(common->dev, "tx0", &dma_tx);
	if (ret) ...

- enable dma channel

	ret = dma_enable(&dma_tx);
	if (ret) ...

- dma transmit DMA_MEM_TO_DEV.

	struct ti_drv_packet_data packet_data;
	
	packet_data.opt1 = val1;
	packet_data.opt2 = val2;
	ret = dma_send(&dma_tx, packet, length, &packet_data);
	if (ret) ..

DMA consumer DMA_DEV_TO_MEM (receive) usage example (based on networking). Note. dma_receive() is sync operation always - it'll start transfer (if required) and will poll for it to complete (or for any previously configured dev2mem transfer to complete):
- get/request dma channel

	struct dma dma_rx;
	ret = dma_get_by_name(common->dev, "rx0", &dma_rx);
	if (ret) ...

- enable dma channel

	ret = dma_enable(&dma_rx);
	if (ret) ...

- dma receive DMA_DEV_TO_MEM.

	struct ti_drv_packet_data packet_data;
	
	len = dma_receive(&dma_rx, (void **)packet, &packet_data);
	if (ret < 0) ...

DMA consumer DMA_DEV_TO_MEM (receive) zero-copy usage example (based on networking). Networking subsystem allows to configure and use few receive buffers (dev2mem), as Networking RX DMA channels usually implemented as streaming interface
- get/request dma channel

	struct dma dma_rx;
	ret = dma_get_by_name(common->dev, "rx0", &dma_rx);
	if (ret) ...
	
	for (i = 0; i < RX_DESC_NUM; i++) {
		ret = dma_prepare_rcv_buf(&dma_rx,
					  net_rx_packets[i],
					  RX_BUF_SIZE);
		if (ret) ...
	}

- enable dma channel

	ret = dma_enable(&dma_rx);
	if (ret) ...

- dma receive DMA_DEV_TO_MEM.

	struct ti_drv_packet_data packet_data;
	void *packet;
	
	len = dma_receive(&dma_rx, &packet, &packet_data);
	if (ret < 0) ..
	
	/* packet - points on buffer prepared by dma_prepare_rcv_buf().
	   process packet*/
	
	- return buffer back to DAM channel
	ret = dma_prepare_rcv_buf(&dma_rx,
				  net_rx_packets[rx_next],
				  RX_BUF_SIZE);