mailbox: imx: Mark PM functions as __maybe_unused

When CONFIG_PM and CONFIG_PM_SLEEP are unset, the following warnings
occur:

drivers/mailbox/imx-mailbox.c:638:12: warning: 'imx_mu_runtime_resume'
defined but not used [-Wunused-function]
638 | static int imx_mu_runtime_resume(struct device *dev)
| ^~~~~~~~~~~~~~~~~~~~~
drivers/mailbox/imx-mailbox.c:629:12: warning: 'imx_mu_runtime_suspend'
defined but not used [-Wunused-function]
629 | static int imx_mu_runtime_suspend(struct device *dev)
| ^~~~~~~~~~~~~~~~~~~~~~
drivers/mailbox/imx-mailbox.c:611:12: warning: 'imx_mu_resume_noirq'
defined but not used [-Wunused-function]
611 | static int imx_mu_resume_noirq(struct device *dev)
| ^~~~~~~~~~~~~~~~~~~
drivers/mailbox/imx-mailbox.c:601:12: warning: 'imx_mu_suspend_noirq'
defined but not used [-Wunused-function]
601 | static int imx_mu_suspend_noirq(struct device *dev)
| ^~~~~~~~~~~~~~~~~~~~

Mark these functions as __maybe_unused, which is the standard procedure
for PM functions.

Fixes: bb2b2624dbe2 ("mailbox: imx: Add runtime PM callback to handle MU clocks")
Signed-off-by: Nathan Chancellor <[e-mail]>
Reviewed-by: Dong Aisheng <[e-mail]>
Signed-off-by: Jassi Brar <[e-mail]>

drivers/mailbox/imx-mailbox.c

···
 };
 MODULE_DEVICE_TABLE(of, imx_mu_dt_ids);
 
-static int imx_mu_suspend_noirq(struct device *dev)
+static int __maybe_unused imx_mu_suspend_noirq(struct device *dev)
 {
 	struct imx_mu_priv *priv = dev_get_drvdata(dev);
 
···
 	return 0;
 }
 
-static int imx_mu_resume_noirq(struct device *dev)
+static int __maybe_unused imx_mu_resume_noirq(struct device *dev)
 {
 	struct imx_mu_priv *priv = dev_get_drvdata(dev);
 
···
 	return 0;
 }
 
-static int imx_mu_runtime_suspend(struct device *dev)
+static int __maybe_unused imx_mu_runtime_suspend(struct device *dev)
 {
 	struct imx_mu_priv *priv = dev_get_drvdata(dev);
 
···
 	return 0;
 }
 
-static int imx_mu_runtime_resume(struct device *dev)
+static int __maybe_unused imx_mu_runtime_resume(struct device *dev)
 {
 	struct imx_mu_priv *priv = dev_get_drvdata(dev);
 	int ret;