iio: adc: imx8qxp: Switch to DEFINE_RUNTIME_DEV_PM_OPS and pm_ptr()

Switching to these newer macros allows the compiler to remove
the unused functions and struct dev_pm_ops if !CONFIG_PM without
the need to mark anything __maybe_unused.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Cai Huoqing <cai.huoqing@linux.dev>
Reviewed-by: Haibo Chen <haibo.chen@nxp.com>
Reviewed-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20220621202719.13644-28-jic23@kernel.org