rtc: rx6110: fix warning with !OF

rx6110_spi_of_match is not used when !OF, leading to a warning:

>> drivers/rtc/rtc-rx6110.c:384:34: warning: 'rx6110_spi_of_match' defined but not used [-Wunused-const-variable=]
     384 | static const struct of_device_id rx6110_spi_of_match[] = {
         |                                  ^~~~~~~~~~~~~~~~~~~

Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/r/20221211215756.54002-1-alexandre.belloni@bootlin.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-rx6110.c

@@ -376,7 +376,7 @@ static const struct spi_device_id rx6110_spi_id[] = {
 };
 MODULE_DEVICE_TABLE(spi, rx6110_spi_id);
 
-static const struct of_device_id rx6110_spi_of_match[] = {
+static const __maybe_unused struct of_device_id rx6110_spi_of_match[] = {
 	{ .compatible = "epson,rx6110" },
 	{ },
 };