media: doc: media/v4l-drivers: Add Qualcomm Camera Subsystem driver document

Add a document to describe Qualcomm Camera Subsystem driver.

Signed-off-by: Todor Tomov <todor.tomov@linaro.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>