media: imx.rst: Provide instructions for the i.MX6DL sabreauto

The i.MX6DL sabreauto has different numbering on the I2C bus and
I2C muxes compared to the i.MX6Q as shown in the kernel log below:

[    5.159423] imx-media: ipu1_csi0_mux:5 -> ipu1_csi0:0
[    5.164618] imx-media: ipu1_csi1_mux:5 -> ipu1_csi1:0
[    5.169953] imx-media: adv7180 4-0021:0 -> ipu1_csi0_mux:4

To avoid confusion, add an entry that shows how to setup the links and
configure the pads that are specific to the i.MX6DL sabreauto.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Steve Longerbeam <slongerbeam@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>