dt-bindings:iio:adc:brcm,iproc-static-adc: txt to yaml conversion

A few questions came up in this one.
1) Why does the txt file document io-channel-ranges as a required property.
   That property is for iio-channel consumers, and this is a provider.
   I have dropped it.
2) The example had an @180a6000 for the ADC but given it uses syscon for
   all access, it doesn't have its own reg etc.  I've dropped that from
   the binding example.

Note this example was lifted directly from bcm-cygnus.dtsi so both
issues are present there as well.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Raveendra Padasalagi <raveendra.padasalagi@broadcom.com>
Link: https://lore.kernel.org/r/20201031184854.745828-33-jic23@kernel.org