IIO channel type and modifiers for CCT and RGBC data

Add iio channel type and modifiers for Correlated Color Temperature (CCT)
and RGBC (red/green/blue/clear) data.
Add CCT and RGBC descriptions to documentation.

Changes:
Revised/condensed RGBC descriptions.

Merge and trivial fix done by Jonathan Cameron.

Signed-off-by: Jon Brenner <jbrenner@taosinc.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>