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

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.net
Signed-off-by: Mark Brown <broonie@kernel.org>