ASoC: SDCA: Add support for Entity 0

ASoC: SDCA: Add support for Entity 0

Within SDCA there is a special Entity called Entity 0 which is used
to hold Function level controls. Whilst Entity 0 isn't a full SDCA
Entity, it is helpful to add an sdca_entity structure for it. This
will allow it to be treated identically in the code that handles
SDCA Controls.

Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.dev>
Link: https://patch.msgid.link/20250205113801.3699902-5-ckeepax@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>