Add board config for Retro Lite CM5

[ginkage]

Description

Community-supported board (myself as the maintainer).

Documentation summary for feature / change

Pretty straightforward: mainline U-Boot, only the vendor kernel supported (no legacy or edge), device tree already present in the kernel branch, common defconfig for RK3588.

How Has This Been Tested?

Built, flashed, ran on the board.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules