docs: firmware-guide: ACPI: Replace dma_request_slave_channel() with dma_request_chan()

DMA controllers enumerated via ACPI should be registered in the system to

provide generic access to their resources. For example, a driver that would

like to be accessible to slave devices via generic API call

dma_request_slave_channel() must register itself at the end of the probe

function like this::

dma_request_chan() must register itself at the end of the probe function like

this::

	err = devm_acpi_dma_controller_register(dev, xlate_func, dw);

	/* Handle the error if it's not a case of !CONFIG_ACPI */

	}

	#endif

dma_request_slave_channel() will call xlate_func() for each registered DMA

controller. In the xlate function the proper channel must be chosen based on

dma_request_chan() will call xlate_func() for each registered DMA controller.

In the xlate function the proper channel must be chosen based on

information in struct acpi_dma_spec and the properties of the controller

provided by struct acpi_dma.

Clients must call dma_request_slave_channel() with the string parameter that

corresponds to a specific FixedDMA resource. By default "tx" means the first

entry of the FixedDMA resource array, "rx" means the second entry. The table

below shows a layout::

Clients must call dma_request_chan() with the string parameter that corresponds

to a specific FixedDMA resource. By default "tx" means the first entry of the

FixedDMA resource array, "rx" means the second entry. The table below shows a

layout::

	Device (I2C0)

	{