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: Lars-Peter Clausen <lars@metafoo.de>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>
Fixes: d7b528ef ("dt: Add bindings documentation for the ADI AXI-SPDIF audio controller")
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Mark Brown <broonie@linaro.org>