adc: stm32mp15: add calibration support

Add support of offset and linear calibration for STM32MP15.
The calibration is performed once at probe. The ADC is set in power on
state for calibration. It remains in this state after calibration,
to give to the kernel the opportunity to retrieve calibration data,
directly from the ADC.

Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>