FROMGIT: media: docs: Document colorimetry class

Add a document for ext control colorimetry class.

Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
(cherry picked from commit 4e4053a2
 git://linuxtv.org/media_tree.git master)
Bug: 175389589
Link: https://lkml.org/lkml/2021/3/23/646



Change-Id: I4aa8a420221ba6a98caa0fbac57aacdfc5e22224
Signed-off-by: Dikshita Agarwal <dikshita@codeaurora.org>