media: intel-ipu3: cio2: mark PM functions as __maybe_unused

When CONFIG_PM is disabled, we get harmless warnings about the
suspend/resume callbacks being unused:

drivers/media/pci/intel/ipu3/ipu3-cio2.c:1993:12: error: 'cio2_resume' defined but not used [-Werror=unused-function]
drivers/media/pci/intel/ipu3/ipu3-cio2.c:1967:12: error: 'cio2_suspend' defined but not used [-Werror=unused-function]

This marks them as __maybe_unused to shut up the warnings.

Fixes: c2a6a07a ("media: intel-ipu3: cio2: add new MIPI-CSI2 driver")

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>