media: imx.rst: Specify the sabreauto variant

Improve the documentation by specifying that the instructions
are related to the i.MX6Q sabreauto variant.

This avoids confusion if someone follows these steps on a i.MX6DL
sabreauto, which has different numbering on the I2C bus and
I2C muxes.

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>