dt-bindings: iio: adc: Add common ADCs properties to a separate file

There are several ADC drivers that depend on the same device tree
bindings. Rather than continue to duplicate the properties, this patch
adds a common adc binding document that can be referenced. For beginning,
only two properties are documented.

Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>