regulator: ltc3589: Fix W=1 build warning when CONFIG_OF=n

Fix below warning when CONFIG_OF=n:

drivers/regulator/ltc3589.c:460:34: warning: ‘ltc3589_of_match’ defined but not used [-Wunused-const-variable=]
  460 | static const struct of_device_id ltc3589_of_match[] = {
      |                                  ^~~~~~~~~~~~~~~~
Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Link: https://lore.kernel.org/r/20200821111449.7cf580f2@xhacker.debianSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/regulator/ltc3589.c

@@ -457,7 +457,7 @@ static const struct i2c_device_id ltc3589_i2c_id[] = {
 };
 MODULE_DEVICE_TABLE(i2c, ltc3589_i2c_id);
 
-static const struct of_device_id ltc3589_of_match[] = {
+static const struct of_device_id __maybe_unused ltc3589_of_match[] = {
 	{
 		.compatible = "lltc,ltc3589",
 		.data = (void *)LTC3589,