dt-bindings: iio: adc: ti,ads7950 binding conversion

Conversion from txt to yaml.  The binding documents that
as not all boards will make use of the ADC channels via a consumer
driver.  It does no harm however, so we will leave it as required.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: David Lechner <david@lechnology.com>
Cc: David Lechner <david@lechnology.com>
Link: https://lore.kernel.org/r/20200830161154.3201-3-jic23@kernel.org