dt-bindings: Add doc for Fastrhino R66S / R68S

Add devicetree binding documentation for the
Lunzn Fastrhino R66S and R68S boards.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Tianling Shen <cnsztl@gmail.com>
Link: https://lore.kernel.org/r/20230506061108.17658-1-cnsztl@gmail.com


Signed-off-by: Heiko Stuebner <heiko@sntech.de>