rtc: ds1390: mark OF related data as maybe unused

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

  drivers/rtc/rtc-ds1390.c:216:34: error: ‘ds1390_of_match’ defined but not used [-Werror=unused-const-variable=]
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230311111226.250922-1-krzysztof.kozlowski@linaro.orgSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-ds1390.c

@@ -213,7 +213,7 @@ static int ds1390_probe(struct spi_device *spi)
 	return res;
 }
 
-static const struct of_device_id ds1390_of_match[] = {
+static const struct of_device_id ds1390_of_match[] __maybe_unused = {
 	{ .compatible = "dallas,ds1390" },
 	{}
 };