Add resolution information to digitizer

[Ofenhed]

This fixes an issue where libinput refuses to accept the QMK digitizer because of missing information.

Description

I'm surprised to not find an issue in the QMK issue tracker, but the digitizer will currently fail with libinput with libinput bug: missing tablet capabilities: resolution. Ignoring this device.. There is more information and a workaround in Evidlo/remarkable_mouse#18.

This PR adds resolution information by adding physical size to the digitizer. The size doesn't really matter, as far as I can tell, but libinput won't play if it's missing.

Note that I'm not at all familiar with HID or USB protocols, but as far as I can tell from HID Usage Tables 1.12 (page 134), my changes seem to conform to the protocol. I have also verified that libinput is happy with the extra information in the descriptor.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout (addition or update)
• Documentation

Issues Fixed or Closed by This PR

This issue had not been reported.

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).