Add constants for GE2/1 RSSI monitoring

Description

Title says it all, this commit adds the required constants to readout the VTRx RSSI on the GE2/1 OptoHybrids.

Related Issue

Relates to #189 (closed) : partial implementation.

How Has This Been Tested?

Tested at p5 on the GE2/1 demonstrator.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• 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.
• All new and existing tests passed.