[media] atmel-isi: update device tree bindings documentation

The original bindings documentation was incomplete (missing pinctrl-names,
missing endpoint node properties) and the example was out of date.

Add the missing information and tidy up the text.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>