How to power a voltage divider with 2.1V
For a photoresistor that varies (for example) from 10 MOhms with no light, to 100 Ohms with bright light, the following voltage divider arrangement would provide a 0-2.1 Volt range as desired, with a 3.3 Volt supply:







simulate this circuit &ndash; Schematic created using CircuitLab

At pitch dark, this would yield 2.1 Volts, while at maximum light, it would be approximately 0 Volts. 

In order to take better advantage of say the 100 Ohm to 1 MOhm range of the photoresistor and consider anything above 1 MOhm as "too dark", R1+R2 can be chosen to total 570 kOhms, instead of the 5.7 MOhms shown.