dt-bindings:iio:adc:ti,twl4030-madc yaml conversion

Conversion from txt to yaml as part of a general move of IIO bindings
to the new format.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Sebastian Reichel <sre@kernel.org>
Link: https://lore.kernel.org/r/20200909175946.395313-3-jic23@kernel.org