Letting the compiler remove these functions when the kernel is built
without CONFIG_PM support is simpler and less error prone than the
use of #ifdef based config guards.

Removing instances of this approach from IIO also stops them being
copied into new drivers.
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20220220181522.541718-5-jic23@kernel.org
Link: https://lore.kernel.org/r/20220604161223.461847-2-jic23@kernel.org