Add DT binding document for Qualcomm Camera subsystem driver.

CC: Rob Herring <robh+dt@kernel.org>
CC: devicetree@vger.kernel.org
Signed-off-by: Todor Tomov <todor.tomov@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>