media: marvell-ccic: mmp: mark PM functions as __maybe_unused

The suspend/resume functions have no callers depending on
configuration, so they must be marked __maybe_unused to
avoid these harmless warnings:

drivers/media/platform/marvell-ccic/mmp-driver.c:347:12: warning:
 'mmpcam_resume' defined but not used [-Wunused-function]
  347 | static int mmpcam_resume(struct device *dev)
      |            ^~~~~~~~~~~~~
drivers/media/platform/marvell-ccic/mmp-driver.c:338:12: warning:
 'mmpcam_suspend' defined but not used [-Wunused-function]
  338 | static int mmpcam_suspend(struct device *dev)
      |            ^~~~~~~~~~~~~~

Fixes: 55cd3452 ("media: marvell-ccic: add support for runtime PM")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>