iio: core: Add optional symbolic label to device attributes

If a label is defined in the device tree for this device add that
to the device specific attributes. This is useful for userspace to
be able to identify an individual device when multiple identical
chips are present in the system.

Tested-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>