ASoC: kirkwood: armada-370-db: mark OF related data as maybe unused

The driver can be compile tested with !CONFIG_OF making certain data
unused:

  sound/soc/kirkwood/armada-370-db.c:137:34: warning: ‘a370db_dt_ids’ defined but not used [-Wunused-const-variable=]
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20201125164452.89239-8-krzk@kernel.orgSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/kirkwood/armada-370-db.c

@@ -134,7 +134,7 @@ static int a370db_probe(struct platform_device *pdev)
 	return devm_snd_soc_register_card(card->dev, card);
 }
 
-static const struct of_device_id a370db_dt_ids[] = {
+static const struct of_device_id a370db_dt_ids[] __maybe_unused = {
 	{ .compatible = "marvell,a370db-audio" },
 	{ },
 };