drivers: adc: adc_ad559x: implement double range option

Optionally write to ad559x register bit which increases ADC input range
from 0V to 2 x Vref.

Create an ADC API per instance since the double range option and thus
`ref_internal` is instance specific.

The functionality has been tested via double range DAC and wiring IO1 and
IO3:
```
$ dac setup ad5593dac 1 12
$ adc ad5593adc resolution 12
$ adc ad5593adc channel id 3
$ dac write_value ad5593dac 1 0
$ adc ad5593adc read 3
read: 1
$ dac write_value ad5593dac 1 1024
$ adc ad5593adc read 3
read: 1021
$ dac write_value ad5593dac 1 2048
$ adc ad5593adc read 3
read: 2044
$ dac write_value ad5593dac 1 3072
$ adc ad5593adc read 3
read: 3069
$ dac write_value ad5593dac 1 4095
$ adc ad5593adc read 3
read: 4091
```

Signed-off-by: Jeppe Odgaard <jeppe.odgaard@prevas.dk>