ASoC: meson: axg-spdifout: select SND_PCM_IEC958

When CONFIG_SND_PCM_IEC958 is disabled, we get a link error for the
new driver:

sound/soc/meson/axg-spdifout.o: In function `axg_spdifout_hw_params':
axg-spdifout.c:(.text+0x650): undefined reference to `snd_pcm_create_iec958_consumer_hw_params'

The other users use 'select', so we should do the same here.

Fixes: 53eb4b7a ("ASoC: meson: add axg spdif output")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Mark Brown <broonie@kernel.org>