dt-bindings: iio: ti,ads8688 yaml conversion

Fairly simple binding.  Most of the changes were filling in information
not previously found in the binding.  I dropped the previous explicit
mention of spi-max-frequency as that is covered by the generic SPI
binding.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Sean Nyekjaer <sean@geanix.com>
Reviewed-by: Rob Herring <robh@kernel.org>