media: atomisp-notes: Add info about sensors v4l2_get_subdev_hostdata() use

Add info about sensors v4l2_get_subdev_hostdata() use, to notes.txt.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>