Facilitate retrieving 12bit ADC values from the nRF52-based V2 #236

The current get_analog() method on the pins returns a 10bit value (0-1023). The nRF52 ADC is 12 bit but I cannot find a way to access them from the current MicroPython APIs on a micro:bit V2. Other MP ports appear to have a machine.ADC with a read_u16().