RAMPS-FD: Thermistor input protection

[cefiar]

Can we implement thermistor protection, as has been implemented in RAMBo v1.2? Given that the circuitry was based on the thread that bobc/nophead & myself all participated in, I don't see any reason we can't also adopt it.

RAMBo PDF of the schematic (showing the protection circuitry in the section marked "Thermistors") can be found on github at https://github.com/ultimachine/RAMBo/blob/master/docs/images/RAMBoSchem.pdf

Note: The OVRef in that design is 4.7V (using a 4.7V zener), whereas it may need to be ~3V with a Due. This would definitely lead to a compatibility difference when trying to use the same shield on either the Due or Mega, though not one that should be too much of a problem to work around.

[cefiar]

Preliminary work rearranging components to make room for the extra protection circuitry completed.