media: renesas-ceu: mark PM functions as __maybe_unused

The PM runtime operations are unused when CONFIG_PM is disabled,
leading to a harmless warning:

drivers/media/platform/renesas-ceu.c:1003:12: error: 'ceu_runtime_suspend' defined but not used [-Werror=unused-function]
 static int ceu_runtime_suspend(struct device *dev)
            ^~~~~~~~~~~~~~~~~~~
drivers/media/platform/renesas-ceu.c:987:12: error: 'ceu_runtime_resume' defined but not used [-Werror=unused-function]
 static int ceu_runtime_resume(struct device *dev)
            ^~~~~~~~~~~~~~~~~~

This adds a __maybe_unused annotation to shut up the warning.

Fixes: 32e5a70d ("media: platform: Add Renesas CEU driver")

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>