Use dev_pm_ops instead of legacy suspend/resume callbacks for IIO drivers.

Note that this patch introduces a few new #ifdef CONFIG_PM_SLEEP around the
suspend and resume callbacks to avoid warnings of unused functions if
CONFIG_PM_SLEEP is not defined.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>