clk: mmp2: fix build without CONFIG_PM

pm_clk_suspend()/pm_clk_resume() are defined as NULL pointers rather than
empty inline stubs without CONFIG_PM:

drivers/clk/mmp/clk-audio.c:402:16: error: called object type 'void *' is not a function or function pointer
pm_clk_suspend(dev);
drivers/clk/mmp/clk-audio.c:411:15: error: called object type 'void *' is not a function or function pointer
pm_clk_resume(dev);

I tried redefining the helper functions, but that caused additional
problems. This is the simple solution of replacing the __maybe_unused
trick with an #ifdef.

Fixes: 725262d29139 ("clk: mmp2: Add audio clock controller driver")
Signed-off-by: Arnd Bergmann <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Stephen Boyd <[e-mail]>

drivers/clk/mmp/clk-audio.c

···
 	return 0;
 }
 
-static int __maybe_unused mmp2_audio_clk_suspend(struct device *dev)
+#ifdef CONFIG_PM
+static int mmp2_audio_clk_suspend(struct device *dev)
 {
 	struct mmp2_audio_clk *priv = dev_get_drvdata(dev);
 
···
 	return 0;
 }
 
-static int __maybe_unused mmp2_audio_clk_resume(struct device *dev)
+static int mmp2_audio_clk_resume(struct device *dev)
 {
 	struct mmp2_audio_clk *priv = dev_get_drvdata(dev);
 
···
 
 	return 0;
 }
+#endif
 
 static const struct dev_pm_ops mmp2_audio_clk_pm_ops = {
 	SET_RUNTIME_PM_OPS(mmp2_audio_clk_suspend, mmp2_audio_clk_resume, NULL)