iio: bu27008: Add illuminance channel

The RGB + IR data can be used to calculate the illuminance value (luxes).
Implement the equation obtained from the ROHM HW colleagues and add a
raw light data channel outputting illuminance values in (nano) Luxes.

Both the read_raw and buffering values are supported, with the limitation
that buffering is only allowed when a suitable scan-mask is used. (RGB+IR,
no clear).

The equation has been developed by ROHM HW colleagues for open air sensor.
Adding any lens to the sensor is likely to impact to used c1, c2, c3
coefficients. Also, the output values have only been tested on BU27008.

According to the HW colleagues, the very same equation should work on
BU27010 as well.

Calculate and output illuminance values from BU27008 and BU27010.

Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Link: https://lore.kernel.org/r/ZTJcOxSb/WHzdN8h@dc78bmyyyyyyyyyyyyydt-3.rev.dnainternet.fi


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>