iio: add power and energy measurement modifiers

iio: add power and energy measurement modifiers

Add new IIO modifiers to support power and energy measurement devices:

Power modifiers:
- IIO_MOD_ACTIVE: Real power consumed by the load
- IIO_MOD_REACTIVE: Power that oscillates between source and load
- IIO_MOD_APPARENT: Magnitude of complex power

Signal quality modifiers:
- IIO_MOD_RMS: Root Mean Square value

Additionally adds:
- IIO_CHAN_INFO_POWERFACTOR: Power factor channel info type for
  representing the ratio of active power to apparent power

These modifiers enable proper representation of power measurement
devices like energy meters and power analyzers.

Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>