MLK-22309-2 media: mxc-mipi-csi2: mark PM functions as __maybe_unused

MLK-22309-2 media: mxc-mipi-csi2: mark PM functions as __maybe_unused

fix build warnings if CONFIG_PM_SLEEP is disabled.

drivers/media/platform/imx8/mxc-mipi-csi2_yav.c:703:12: warning: 'mipi_csi2_pm_resume' defined but not used [-Wunused-function]
 static int mipi_csi2_pm_resume(struct device *dev)
            ^~~~~~~~~~~~~~~~~~~
drivers/media/platform/imx8/mxc-mipi-csi2_yav.c:685:12: warning: 'mipi_csi2_pm_suspend' defined but not used [-Wunused-function]
 static int mipi_csi2_pm_suspend(struct device *dev)
            ^~~~~~~~~~~~~~~~~~~~

Signed-off-by: Robby Cai <robby.cai@nxp.com>
Reviewed-by: Sandor Yu <Sandor.yu@nxp.com>