Add hwdb entry for proximity sensor
We don't have dmi so upstream's hwdb won't cut it and setting via DT is under discussion upstream:
https://gitlab.freedesktop.org/hadess/iio-sensor-proxy/merge_requests/298
has the references.
The current value of 220 works well for Librem5 and devkit.
The hwdb is refresshed on package installation via a dpkg trigger for udev.