ASoC: Add support for the Analog Devices AXI-SPDIF driver

This patch adds a ASoC driver for the AXI-SPDIF softcore. The core implements a
simple SPDIF transmitter and is used on some Analog Devices' reference designs
for various FPGA platforms. For now the driver only support the PL330 as the the
DMA controller.

The driver uses the generic PCM dmaengine driver for its PCM. The only
restriction is that we need to set the SND_DMAENGINE_PCM_FLAG_NO_RESIDUE flag as
the dmaengine driver for the DMA core (PL330) that is used with this core has no
residue reporting capabilities yet. This will be fixed in the future though.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>