ASoC: core: Add common helper to parse aux devs from device tree

simple-card.c and meson-card-utils.c use pretty much the same
helper function to parse auxiliary devices from the device tree.

Make it easier for other drivers to parse these from the device tree
as well by adding a shared helper function to soc-core.c.

snd_soc_of_parse_aux_devs() is pretty much a copy of
meson_card_add_aux_devices() from meson-card-utils.c
with two minor changes:

  - Make property name configurable as parameter
  - Change dev_err() message slightly for consistency with other
    error messages in soc-core.c
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lore.kernel.org/r/20200801100257.22658-1-stephan@gerhold.netSigned-off-by: Mark Brown <broonie@kernel.org>

include/sound/soc.h

@@ -1331,6 +1331,7 @@ void snd_soc_of_parse_audio_prefix(struct snd_soc_card *card,
 
 int snd_soc_of_parse_audio_routing(struct snd_soc_card *card,
 				   const char *propname);
+int snd_soc_of_parse_aux_devs(struct snd_soc_card *card, const char *propname);
 unsigned int snd_soc_of_parse_daifmt(struct device_node *np,
 				     const char *prefix,
 				     struct device_node **bitclkmaster,

sound/soc/soc-core.c

@@ -2827,6 +2827,37 @@ int snd_soc_of_parse_audio_routing(struct snd_soc_card *card,
 }
 EXPORT_SYMBOL_GPL(snd_soc_of_parse_audio_routing);
 
+int snd_soc_of_parse_aux_devs(struct snd_soc_card *card, const char *propname)
+{
+	struct device_node *node = card->dev->of_node;
+	struct snd_soc_aux_dev *aux;
+	int num, i;
+
+	num = of_count_phandle_with_args(node, propname, NULL);
+	if (num == -ENOENT) {
+		return 0;
+	} else if (num < 0) {
+		dev_err(card->dev, "ASOC: Property '%s' could not be read: %d\n",
+			propname, num);
+		return num;
+	}
+
+	aux = devm_kcalloc(card->dev, num, sizeof(*aux), GFP_KERNEL);
+	if (!aux)
+		return -ENOMEM;
+	card->aux_dev = aux;
+	card->num_aux_devs = num;
+
+	for_each_card_pre_auxs(card, i, aux) {
+		aux->dlc.of_node = of_parse_phandle(node, propname, i);
+		if (!aux->dlc.of_node)
+			return -EINVAL;
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(snd_soc_of_parse_aux_devs);
+
 unsigned int snd_soc_of_parse_daifmt(struct device_node *np,
 				     const char *prefix,
 				     struct device_node **bitclkmaster,