`humility gpio` should not show values for pins in analog mode on STM32

STM32 parts offer an "analog mode" pin configuration that disconnects the input Schmitt trigger from the pin, to avoid disturbing weak analog signals or hanging out in metastable ranges.

STM32H7 reference manual (RM0433 rev 7) section 11.3.13 "Analog Configuration" provides the skinny here, but the short version is

1. In Analog mode the "input" always reads as zero and is unconnected to the pin.
2. Many, many pins reset in analog mode. Nearly all of them in fact.

When Humility displays gpio state for pins, it would ideally not display "0" for pins that are in analog mode, as it has misled people into thinking that the connected nets are being held low, when in fact it indicates nothing of the sort.

This will require changes outside Humility -- the GPIO driver doesn't expose this information, in particular, because applications don't typically care. They configure their pins and then do the things, there's no need to check whether it has mysteriously reverted to analog at each step. But, for applications like Hiffy, we could add an operation to read out the mask of which pins are providing legit input values, and which are disconnected, say.