dt-binding: iio: Add optional label property

This optional property defines a symbolic name for the device.
This helps to distinguish between more than one iio device
of the same type.

Suggested-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Phil Reid <preid@electromag.com.au>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>