[media] atmel-isc: mark PM functions as __maybe_unused

The newly added atmel-isc driver uses SET_RUNTIME_PM_OPS() to
refer to its suspend/resume functions, causing a warning when
CONFIG_PM is not set:

media/platform/atmel/atmel-isc.c:1477:12: error: 'isc_runtime_resume' defined but not used [-Werror=unused-function]
media/platform/atmel/atmel-isc.c:1467:12: error: 'isc_runtime_suspend' defined but not used [-Werror=unused-function]

This adds __maybe_unused annotations to avoid the warning without
adding an error-prone #ifdef around it.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>