Because this must be treated like an in-use pin for all other purposes,
unfortunately a special case must be added in shared-bindings.
Multiple AnalogIn objects for VOLTAGE_MONITOR can be created (because
in use tracking isn't working) but this causes no harm.
Testing performed: Read the monitor, then imported wifi. When the
pin state was insufficiently restored, the second step would fail
with debug messages about do_ioctl timeout.
```
import analogio, board
a = analogio.AnalogIn(board.VOLTAGE_MONITOR)
print(a.value)
import wifi
```
Closes: #7020
This reduces power consumption during true deep sleep.
In my measurements with ppk2 and a program that _irrevocably_ entered
deep sleep (no time alarm or pin alarm), power usage as measured on a
ppk2 decreased from ~10mA to ~1mA.
.. the value actually needs to be enforced each time the STA or AP
is enabled, because internally there's a call to cyw43_wifi_pm with the
library's defaut power management value, not ours.
Add a getter, though it only returns our idea of what the power
management register is set to, it doesn't read out from the actual
hardware, sadly.
