boards: arduino: fix ADC assignments for SAMD21-based boards

The ADC pin assignments for the Arduino Zero, MKR Zero and Nano 33 IoT
boards were missing in the device tree files, even though the boards
advertise ADC functionality. This was highlighted recently by commit
162f7287 which made it a build error.

This patch fixes the issue by adding the detailed pinout assignments
for the analog pins on these boards.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>