dt-bindings: iio: Add ltc2688 documentation

Document the LTC2688 devicetree properties.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220225130129.69-4-nuno.sa@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>