dt-bindings:iio:adc:qcom,spmi-iadc: txt to yaml format conversion.

Straight forward conversion.  Not heard from Ivan in a while so if the
email bounces, I'll change the maintainer to myself for this binding unless
anyone else comes forwards.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201031184854.745828-37-jic23@kernel.org