tests: adc_api: Add configurations for nRF5 boards

This adds to the adc_api test board-specific configurations
for the following boards:
- nrf51_pca10028
- nrf52_pca10040
- nrf52840_pca10056

"adc" is indicated as supported in the yaml files for these boards
so that they are included in sanitycheck.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>