fix(boards): Properly note ZMK compat for xiao_rp2040/zmk variant

[petejohanson]

Add missing board Kconfig file for properly noting ZMK board compat and enabling boot mode retention for the xiao_rp2040//zmk board variant.

Fixes #3325

PR check-list

• Branch has a clean commit history
• Additional tests are included, if changing behaviors/core code that is testable.
• Proper Copyright + License headers added to applicable files (Generally, we stick to "The ZMK Contributors" for copyrights to help avoid churn when files get edited)
• Pre-commit used to check formatting of files, commit messages, etc.
• Includes any necessary documentation changes.