iio: light: ltr390: Implement runtime PM support

iio: light: ltr390: Implement runtime PM support

Implement runtime power management for the LTR390 sensor. The device
autosuspends after 1s of idle time, reducing current consumption from
100 µA in active mode to 1 µA in standby mode as per the datasheet.

Ensure that interrupts continue to be delivered with runtime PM.
Since the LTR390 cannot be used as a wakeup source during runtime
suspend, therefore increment the runtime PM refcount when enabling
events and decrement it when disabling events or powering down.
This prevents event loss while still allowing power savings when IRQs
are unused.

Signed-off-by: Akshay Jindal <akshayaj.lkd@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>