iio: adc: Add rtq6056 support

Add Richtek rtq6056 supporting.

It can be used for the system to monitor load current and power with 16-bit
resolution.

Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/1658242365-27797-3-git-send-email-u0084500@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>