dt-bindings: iio: adc: ti,adc108s102 yaml conversion

Very simple conversion of spi device with reference supply.

Added the #io-channel-cells property to allow for consumers.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Link: https://lore.kernel.org/r/20200809111753.156236-4-jic23@kernel.org