drivers: adc: add API to support calibration

Add a flag to the sequence structure that tells the driver it should
calibrate the ADC prior to initiating the sample.

Implement this for nRF SAADC.  The implementation supports the
workarounds for PAN-86 and PAN-178.

Relates-to: issue #11922
Signed-off-by: Peter A. Bigot <pab@pabigot.com>