dt-bindings:iio:adc:adi,ad7949 yaml conversion

Simple SPI driver. I've added the #io-channel-cells
as an optional parameter to allow use of this device as a provider
of ADC capabilities to other devices.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Charles-Antoine Couret <charles-antoine.couret@essensium.com>
Link: https://lore.kernel.org/r/20200909175946.395313-15-jic23@kernel.org