dt-bindings: iio: adc: maxim,max1118 yaml conversion

Simple device with a simple conversion.  Special handling needed
for the max1118 which is the only supported part that has an external
reference voltage.

Cc: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>