spi: sc18is602: Mark OF related data as maybe unused

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

  drivers/spi/spi-sc18is602.c:318:34: error: ‘sc18is602_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/20230310222857.315629-15-krzysztof.kozlowski@linaro.orgSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-sc18is602.c

@@ -315,7 +315,7 @@ static const struct i2c_device_id sc18is602_id[] = {
 };
 MODULE_DEVICE_TABLE(i2c, sc18is602_id);
 
-static const struct of_device_id sc18is602_of_match[] = {
+static const struct of_device_id sc18is602_of_match[] __maybe_unused = {
 	{
 		.compatible = "nxp,sc18is602",
 		.data = (void *)sc18is602