The current implementation hard codes the two supported channels so that
"tx" is always 0 and "rx" is always 1. This is because there has been no
suitable way in ACPI to name resources.

With _DSD device properties we can finally do this:

	Device (SPI1) {
	    Name (_CRS, ResourceTemplate () {
	        ...
	        FixedDMA (0x0000, 0x0000, Width32bit)
	        FixedDMA (0x0001, 0x0001, Width32bit)
	    })

	    Name (_DSD, Package () {
	        ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
	        Package () {
	            Package () {"dma-names", Package () {"tx", "rx"}}
	        },
	    })
	}

The names "tx" and "rx" now provide index of the FixedDMA resource in
question.

Modify acpi_dma_request_slave_chan_by_name() so that it looks for
"dma-names" property first and only then fall back using hardcoded indices.

The DT "dma-names" binding that we reuse for ACPI is documented in
Documentation/devicetree/bindings/dma/dma.txt.
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>