Hi Aloyse,
The following enum compiles and should work and produce a 2V internal voltage reference, which could be a fair compromise near to 2.5 V.
ADC->INPUTCTRL.bit.GAIN = ADC_INPUTCTRL_GAIN_2X_Val; // Gain Factor Selection
ADC->REFCTRL.bit.REFSEL = ADC_REFCTRL_REFSEL_INT1V_Val; // 2.0V voltage reference
Furthermore the 1.1 V reference is a high stability precision bandgap reference. According to Table 36.35 variation over wide temp range is +/- 20 mV. The 2.2 V reference would have a tolerance of +/- 40 mV, or +/- 2%, which is a remarkable level of precision.
This could be further refined by linear interpolation of data stored in the chip, by applying data of table 36.38 in reverse, e.g. using data of an ext. temp sensor to correct bandgap voltage, but requires further hard work.