media: atomisp: Add a notes.txt file

Add a files documenting what I've learned about the driver while
working on various cleanups.

Link: https://lore.kernel.org/linux-media/20220615205037.16549-41-hdegoede@redhat.com


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>