fix ADC management for pure analog on GIGA

[Rocketct]

This PR allows to fix the management of the pure analogs on GIGA, the issue was that the pure analog pins have not been considered on the g_APinDescription array, this results on miss an offset of 4 elements when a g_AAnalogPinDescription's pin was select in the analogRead() in wiring_Analog.cpp.
The consequence is that the macros involved in Pin and ADC selection, could potentially identify an ADC assigned to the pure analog pins as the ADC channel for all those analog listed after the pin A11 in g_AAnalogPinDescription array in the variant.cpp and than read the ADC channel of the pure analog pins instead of the correct one (e. g. A8 for A12, A9 for A13 and so on and so forth).

The Fix split the two kind of analog pin in order that the correct offset is provided during reading time and manage the pure analog and the other analogs pin as a different entities inside the pins management.

This PR fixes #758

[CLAassistant]

All committers have signed the CLA.

[njadNissi]

I have the most updated version of this library in my ArduinoIDE, I checked that all the code is the same. however calling anologRead(A8) or of any of the pureAnalogPins still doesn't work. could you provide an example sketch to show how it should be used?