ASoC: gtm601: mark OF related data as maybe unused

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

  sound/soc/codecs/gtm601.c:90:34: warning: ‘gtm601_codec_of_match’ defined but not used [-Wunused-const-variable=]
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20201125164452.89239-4-krzk@kernel.orgSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/gtm601.c

@@ -87,7 +87,7 @@ static int gtm601_platform_probe(struct platform_device *pdev)
 			(struct snd_soc_dai_driver *)dai_driver, 1);
 }
 
-static const struct of_device_id gtm601_codec_of_match[] = {
+static const struct of_device_id gtm601_codec_of_match[] __maybe_unused = {
 	{ .compatible = "option,gtm601", .data = (void *)&gtm601_dai },
 	{ .compatible = "broadmobi,bm818", .data = (void *)&bm818_dai },
 	{},