iio: adc: at91-sama5d2_adc: add runtime pm support

Add runtime PM support by disabling/enabling ADC's peripheral clock.
On simple conversion the ADC's clock is kept enabled just while the
conversion is in progress. This includes also temperature conversion.
For triggered buffers and touch conversions the ADC clock is kept enabled
while the triggered buffers or touch are enabled. Along with it removed
the __maybe_unused on suspend() and resume() ops as the dev_pm_ops
object members are now filled with SYSTEM_SLEEP_PM_OPS().
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20220803102855.2191070-20-claudiu.beznea@microchip.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>