iio: light: introduce si1133

e-mail received from Silicon Lab to confirm that the licensing
isn't a problem.

"
Dear Maxime Roussin-Belanger,

The LUX calculation code only works with Si1133.
As long as the software is used with Silicon Lab's sensor product,
I don't see any problem.

Regards,
Tony
"

Signed-off-by: Maxime Roussin-Bélanger <maxime.roussinbelanger@gmail.com>
Reviewed-by: Jean-Francois Dagenais <jeff.dagenais@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>