DT binding documentation for ISC driver.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Songjun Wu <songjun.wu@microchip.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>