External ADC pins (ADC1 and ADC2) are not yet readable from Linux sysfs (hopefully it should be supported in the coming releases).
Currently it is only possible to read some internal ADC functions (voltage, temperature) from “/sys/bus/platform/devices/pm8xxx-adc/”.
It is still possible to read ADC1 and ADC2 pins from external AT port (USB or UART) using the following AT commands: ADC1AT!MADC?5
!MADC: <value>
OKADC2AT!MADC?6
!MADC: <value>
OK