Raspberry PI 4: type, memory, processor are "unknown"

My 2GB Raspberry Pi 4 is reported as:

```
>>> getInfo()['revision']
{'type': 'unknown', 'memory': 'unknown', 'processor': 'unknown', 'revision': 1.1, 'manufacturer': 'Sony UK', 'code': 'b03111'}
```

The details are actually documented at https://www.raspberrypi.org/documentation/hardware/raspberrypi/revision-codes/README.md