regulator: stm32-pwr: Fix W=1 build warning when CONFIG_OF=n

Fix below warning when CONFIG_OF=n:

drivers/regulator/stm32-pwr.c:169:34: warning: ‘stm32_pwr_of_match’ defined but not used [-Wunused-const-variable=]
  169 | static const struct of_device_id stm32_pwr_of_match[] = {
      |                                  ^~~~~~~~~~~~~~~~~~
Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Link: https://lore.kernel.org/r/20200821111726.38e0e746@xhacker.debianSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/regulator/stm32-pwr.c

@@ -166,7 +166,7 @@ static int stm32_pwr_regulator_probe(struct platform_device *pdev)
 	return ret;
 }
 
-static const struct of_device_id stm32_pwr_of_match[] = {
+static const struct of_device_id __maybe_unused stm32_pwr_of_match[] = {
 	{ .compatible = "st,stm32mp1,pwr-reg", },
 	{},
 };