ASoC: ti: j721e-evm: Convert the audio domain IDs to enum

Convert the J721E_AUDIO_DOMAIN_* from defines to enum to make it possible
to extend the number of domains in the future.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@gmail.com>
Link: https://lore.kernel.org/r/20210717122820.1467-4-peter.ujfalusi@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>