drivers: adc: Add ADC xmc4xxx drivers

The ADC module has four conversion groups, each one is set up as a zephyr
device. The start-up calibration is initiated globally for all groups
and it is run in each device init function. The ADC module supports post
calibration per group. Post calibration is run automatically after each
group acquires the samples.

Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>