drivers: adc: stm32: add support for differential mode

Differential mode support consists of:
- If differential mode is supported by the underlying hardware AND at
  least one differential channel is enabled in the devicetree for this
  ADC instance, then perform a differential mode calibration in addition
  to the usual single ended calibration during initialisation.
- Set channels to the appropriate differential or single ended mode
  during channel setup.

Currently the N6 series is not supported even though the underlying
hardware supports differential mode, due to complications in the
calibration procedure.

Signed-off-by: Matt Rodgers <mrodgers@witekio.com>
Co-authored-by: Guillaume Gautier <guillaume.gautier-ext@st.com>