Simple binding conversion of this SPI ADC binding, with reference
voltage.

Added the optional property #io-channel-cells to allow for
consumers of channels if that makes sense for a given board.
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Akinobu Mita <akinobu.mita@gmail.com>
Link: https://lore.kernel.org/r/20200809111753.156236-7-jic23@kernel.org