iio: adc128s052: Add pin-compatible IDs

The datasheets for ADC122S021 and ADC124S021 list two more
pin-compatible alternatives for each device.

Add their IDs as compatible strings.

Suggested-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Javier Arteaga <javier@emutex.com>
Signed-off-by: Dan O'Donovan <dan@emutex.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>