iio: imx8qxp-adc: mark PM functions as __maybe_unused

Without CONFIG_PM_SLEEP, the runtime suspend/resume functions
are unused, producing a warning:

drivers/iio/adc/imx8qxp-adc.c:433:12: error: 'imx8qxp_adc_runtime_resume' defined but not used [-Werror=unused-function]
  433 | static int imx8qxp_adc_runtime_resume(struct device *dev)
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/iio/adc/imx8qxp-adc.c:419:12: error: 'imx8qxp_adc_runtime_suspend' defined but not used [-Werror=unused-function]
  419 | static int imx8qxp_adc_runtime_suspend(struct device *dev)
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~

Mark them as __maybe_unused to shut up the compiler.

Fixes: 1e23dcaa ("iio: imx8qxp-adc: Add driver support for NXP IMX8QXP ADC")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Cai Huoqing <caihuoqing@baidu.com>
Link: https://lore.kernel.org/r/20211013144338.2261316-1-arnd@kernel.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>