ASoC: rk3328: mark OF related data as maybe unused

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

  sound/soc/codecs/rk3328_codec.c:502:34: warning: ‘rk3328_codec_of_match’ defined but not used [-Wunused-const-variable=]
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20201125164452.89239-6-krzk@kernel.orgSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/rk3328_codec.c

@@ -499,7 +499,7 @@ static int rk3328_platform_probe(struct platform_device *pdev)
 					       ARRAY_SIZE(rk3328_dai));
 }
 
-static const struct of_device_id rk3328_codec_of_match[] = {
+static const struct of_device_id rk3328_codec_of_match[] __maybe_unused = {
 		{ .compatible = "rockchip,rk3328-codec", },
 		{},
 };