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

Fix below warning when CONFIG_OF=n:

drivers/regulator/pwm-regulator.c:393:34: warning: ‘pwm_of_match’ defined but not used [-Wunused-const-variable=]
  393 | static const struct of_device_id pwm_of_match[] = {
      |                                  ^~~~~~~~~~~~
Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Link: https://lore.kernel.org/r/20200821111658.59a7218b@xhacker.debianSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/regulator/pwm-regulator.c

@@ -390,7 +390,7 @@ static int pwm_regulator_probe(struct platform_device *pdev)
 	return 0;
 }
 
-static const struct of_device_id pwm_of_match[] = {
+static const struct of_device_id __maybe_unused pwm_of_match[] = {
 	{ .compatible = "pwm-regulator" },
 	{ },
 };