dt-bindings:iio:adc:sprd,sc2720-adc yaml conversion.

I changed the name to reflect a specific part in line with normal
naming conventions. If there is a particularly strong reason to
keep the wild cards let me know.

Otherwise this was a fairly simple conversion as part of converting
all the IIO bindings to yaml.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Orson Zhai <orsonzhai@gmail.com>
Cc: Baolin Wang <baolin.wang7@gmail.com>
Cc: Chunyan Zhang <zhang.lyra@gmail.com>
Link: https://lore.kernel.org/r/20200909175946.395313-7-jic23@kernel.org