ASoC: axi: Fix ADI AXI SPDIF specification

The specification requires compatible = "adi,axi-spdif-1.00.a" but
driver and example and file name indicate "adi,axi-spdif-tx-1.00.a".
Change the specification to match the implementation.
Acked-by: NLars-Peter Clausen <lars@metafoo.de>
Reviewed-by: NMichal Simek <michal.simek@xilinx.com>
Fixes: d7b528ef ("dt: Add bindings documentation for the ADI AXI-SPDIF audio controller")
Signed-off-by: NAndreas Färber <afaerber@suse.de>
Signed-off-by: NMark Brown <broonie@linaro.org>

Documentation/devicetree/bindings/sound/adi,axi-spdif-tx.txt

 ADI AXI-SPDIF controller
 
 Required properties:
- - compatible : Must be "adi,axi-spdif-1.00.a"
+ - compatible : Must be "adi,axi-spdif-tx-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