mailbox: imx: Mark PM functions as __maybe_unused

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 <natechancellor@gmail.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>