drivers: sensor: Add STM32 VREF+ sensor

Add VREF+ sensor driver and DT node definition.

This driver allows determining the actual voltage applied to an SoC's
VREF+ pin, by comparing the VREFINT internal bandgap voltage reference
with its factory calibration data.

In packages where VREF+ is bonded to VDDA, this permits direct measurement
of VDDA voltage.

Signed-off-by: Kenneth J. Miller <ken@miller.ec>