iio: Add ABI documentation for illuminance raw and scale values in light

This patchset adds ABI documentation for the following attributes:

in_illuminance_scale, used atleast once in al3320a staging/iio/light/
in_illuminance_calibscale, used atleast once in cm32181
in_illuminance_input, used in cm3232 at least once
in_illuminance_raw used atleast once in al3320a
in_illuminance_clear_raw and in_illuminance_ir_raw exposed by
gp2ap020a00f with modifiers IIO_MOD_LIGHT_CLEAR and
IIO_MOD_LIGHT_IR respectively.

Signed-off-by: Darshana Padmadas <darshanapadmadas@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>