regulator: mt6397-regulator: Mark OF related data as maybe unused

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

  drivers/regulator/mt6397-regulator.c:400:34: error: ‘mt6397_of_match’ defined but not used [-Werror=unused-const-variable=]
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20230310214553.275450-4-krzysztof.kozlowski@linaro.orgSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/regulator/mt6397-regulator.c

@@ -397,7 +397,7 @@ static const struct platform_device_id mt6397_platform_ids[] = {
 };
 MODULE_DEVICE_TABLE(platform, mt6397_platform_ids);
 
-static const struct of_device_id mt6397_of_match[] = {
+static const struct of_device_id mt6397_of_match[] __maybe_unused = {
 	{ .compatible = "mediatek,mt6397-regulator", },
 	{ /* sentinel */ },
 };