ARM: dts: stm32: add vrefint support to adc2 on stm32mp15

Set STM32 ADC2 as a consumer of BSEC on STM32MP15, to retrieve
vrefint calibration data saved in OTP.

During the calibration process vrefp is set to 3.3V and the data
acquired is saved to the OTP. This data is used by the ADC driver
to calculated the actual value of vrefp according to the formula:
vrefp = 3.3 x vrefint_cal / vrefint_data

The vrefint channel provides the actual value of vrefp,
which can be used to correct ADC acquisition data.

Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>