dt-bindings: iio: adc: ti,adc161s626 yaml conversion.

Simple conversion for this ADC driver.  Note that I haven't put
limits on the spi-max-sampling-frequency because the adc161s626
doesn't state one clearly defined value.

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>
Acked-by: Matt Ranostay <matt.ranostay@konsulko.com>
Cc: Matt Ranostay <matt.ranostay@konsulko.com>
Link: https://lore.kernel.org/r/20200809111753.156236-6-jic23@kernel.org