Letting the compiler remove these functions when the kernel is built
without one or  more of CONFIG_PM/CONFIG_PM_SLEEP 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>
Cc: Crt Mori <cmo@melexis.com>
Reviewed-by: Crt Mori <cmo@melexis.com>
Reviewed-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20220130193147.279148-43-jic23@kernel.org