iio: light: cm3605: mark PM functions as __maybe_unused

When CONFIG_PM_SLEEP is disabled, we get a harmless warning

drivers/iio/light/cm3605.c:292:12: error: 'cm3605_pm_resume' defined but not used [-Werror=unused-function]
drivers/iio/light/cm3605.c:281:12: error: 'cm3605_pm_suspend' defined but not used [-Werror=unused-function]

Marking the functions as possibly unused avoids the warning without
needing to add an #ifdef.

Fixes: 8afa505c

 ("iio: light: add driver for Capella CM3605")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>