[media] media: adv7604: fix bindings inconsistency for default-input

The text states that default-input is an endpoint property, but in the
example it is a device property.

The default input is a property of the chip, not of a particular port, so
the example makes more sense.

Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>