ADI AXI-SPDIF controller

Required properties:
 - compatible : Must be "adi,axi-spdif-1.00.a"
 - reg : Must contain SPDIF core's registers location and length
 - clocks : Pairs of phandle and specifier referencing the controller's clocks.
   The controller expects two clocks, the clock used for the AXI interface and
   the clock used as the sampling rate reference clock sample.
 - clock-names: "axi" for the clock to the AXI interface, "ref" for the sample
   rate reference clock.
 - dmas: Pairs of phandle and specifier for the DMA channel that is used by
   the core. The core expects one dma channel for transmit.
 - dma-names : Must be "tx"

For more details on the 'dma', 'dma-names', 'clock' and 'clock-names' properties
please check:
	* resource-names.txt
	* clock/clock-bindings.txt
	* dma/dma.txt

Example:

	spdif: spdif@0x77400000 {
		compatible = "adi,axi-spdif-tx-1.00.a";
		reg = <0x77600000 0x1000>;
		clocks = <&clk 15>, <&audio_clock>;
		clock-names = "axi", "ref";
		dmas = <&ps7_dma 0>;
		dma-names = "tx";
	};

Add bindings documentation for the ADI AXI-SPDIF audio controller
dt: Add bindings documentation for the ADI AXI-SPDIF audio controller

This patch adds the devicetree documentation for the ADI AXI-SPDIF audio
controller. The controller has:
 * One set of memory mapped register
 * Two clocks, one for the memory mapped register interface, one used as the
   audio reference clock
 * A DMA interface for the transmit data

Signed-off-by: Lars-Peter Clausen <3318dc5ce3e4fb7c28a0b841b6801c884e1d0896@metafoo.de>
Cc: Rob Herring <aa00819e304141d872f2e4b1c4be422ea3abbfe1@calxeda.com>
Cc: Pawel Moll <9508d67e84a03f69673bee3bb7ec593a47efeb0d@arm.com>
Cc: Mark Rutland <1db9dd262beaa64f35756dd39531d9747b7778bb@arm.com>
Cc: Stephen Warren <5ef2a23ba3aff51d1cfc8c113c1ec34b608b3b13@wwwdotorg.org>
Cc: Ian Campbell <ea71a0dcef14058f83aacb9a32bd3c02e11f589b@hellion.org.uk>
Cc: eb682e570f95b554c8a7dc560ee59b91b930a718@vger.kernel.org
Signed-off-by: Mark Brown <b51b9a92386687a9ac927cebfa0f978adeb8cea5@linaro.org>
