iio: core: Add IIO_EV_INFO_SCALE to event info

iio: core: Add IIO_EV_INFO_SCALE to event info

Implement support for IIO_EV_INFO_SCALE in the internal enum
iio_event_info to allow proper ABI compliance.

This allows drivers (like the ADXL345) to expose event scale
attributes using the standard IIO ABI rather than manual
device attributes.

Signed-off-by: Taha Ed-Dafili <0rayn.dev@gmail.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>