drivers: dac: ad559x: implement double range option

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

The functionality has been tested on AD5593r.
The voltage at 4095 has been measured to 5.01 volts.

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