boards/arduino-zero: configure ADC channels

[aabadie]

Contribution description

This PR adds the periph_adc features to the arduino-zero board and configure the 6 ADC channels exposed on the board.

Tested with success using the tests/periph_adc application and a potentiometer plugged on each Ax pins.

Testing procedure

Use the tests/periph_adc application and plug a potentiometer on each Ax pins.

Issues/PRs references

Required by #12523

[fjmolinas]

@aabadie can you provide test output int he PR?

[aabadie]

Output of tests/periph_adc:

Potentiometer connected to A0

2019-10-22 13:28:26,637 # ADC_LINE(0): 37
2019-10-22 13:28:26,639 # ADC_LINE(1): 900
2019-10-22 13:28:26,640 # ADC_LINE(2): 1023
2019-10-22 13:28:26,642 # ADC_LINE(3): 848
2019-10-22 13:28:26,644 # ADC_LINE(4): 1023
2019-10-22 13:28:26,646 # ADC_LINE(5): 1023

Potentiometer connected to A1

2019-10-22 13:30:05,994 # ADC_LINE(0): 1023
2019-10-22 13:30:05,995 # ADC_LINE(1): 36
2019-10-22 13:30:05,997 # ADC_LINE(2): 1023
2019-10-22 13:30:05,999 # ADC_LINE(3): 840
2019-10-22 13:30:06,001 # ADC_LINE(4): 951
2019-10-22 13:30:06,002 # ADC_LINE(5): 1023

Potentiometer connected to A2

2019-10-22 13:30:58,687 # ADC_LINE(0): 1023
2019-10-22 13:30:58,688 # ADC_LINE(1): 1023
2019-10-22 13:30:58,690 # ADC_LINE(2): 36
2019-10-22 13:30:58,692 # ADC_LINE(3): 746
2019-10-22 13:30:58,694 # ADC_LINE(4): 1014
2019-10-22 13:30:58,695 # ADC_LINE(5): 960

Potentiometer connected to A3

2019-10-22 13:31:35,647 # ADC_LINE(0): 1023
2019-10-22 13:31:35,649 # ADC_LINE(1): 1023
2019-10-22 13:31:35,650 # ADC_LINE(2): 1023
2019-10-22 13:31:35,652 # ADC_LINE(3): 36
2019-10-22 13:31:35,654 # ADC_LINE(4): 1023
2019-10-22 13:31:35,656 # ADC_LINE(5): 1023

Potentiometer connected to A4

2019-10-22 13:31:59,383 # ADC_LINE(0): 1023
2019-10-22 13:31:59,385 # ADC_LINE(1): 1023
2019-10-22 13:31:59,387 # ADC_LINE(2): 1023
2019-10-22 13:31:59,388 # ADC_LINE(3): 841
2019-10-22 13:31:59,390 # ADC_LINE(4): 37
2019-10-22 13:31:59,392 # ADC_LINE(5): 1023

Potentiometer connected to A5

2019-10-22 13:32:21,819 # ADC_LINE(0): 1023
2019-10-22 13:32:21,820 # ADC_LINE(1): 1023
2019-10-22 13:32:21,822 # ADC_LINE(2): 1023
2019-10-22 13:32:21,824 # ADC_LINE(3): 832
2019-10-22 13:32:21,825 # ADC_LINE(4): 876
2019-10-22 13:32:21,827 # ADC_LINE(5): 37

[aabadie]

ping @fjmolinas

[fjmolinas]

ACK, trusting @aabadie test output!