Add DD {LED,RGB}_MATRIX_DEFAULT_FLAGS support

[zvecr]

Description

• Adds DD config for RGB_MATRIX_DEFAULT_FLAGS and LED_MATRIX_DEFAULT_FLAGS
• Extends "duplicates default" check to cover new option
• Migrates keyboards/xelus/valor/rev2/rev2.c init code to DD

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

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).