iio: adc: rcar-gyroadc: mark OF related data as maybe unused

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

  drivers/iio/adc/rcar-gyroadc.c:286:34: error: ‘rcar_gyroadc_child_match’ defined but not used [-Werror=unused-const-variable=]
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230311111457.251475-1-krzysztof.kozlowski@linaro.orgSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/rcar-gyroadc.c

@@ -283,7 +283,7 @@ static const struct of_device_id rcar_gyroadc_match[] = {
 
 MODULE_DEVICE_TABLE(of, rcar_gyroadc_match);
 
-static const struct of_device_id rcar_gyroadc_child_match[] = {
+static const struct of_device_id rcar_gyroadc_child_match[] __maybe_unused = {
 	/* Mode 1 ADCs */
 	{
 		.compatible	= "fujitsu,mb88101a",