dt-bindings: Add doc for Xunlong OrangePi R1 Plus LTS

Add devicetree binding documentation for the
Xunlong OrangePi R1 Plus LTS.

Signed-off-by: Tianling Shen <cnsztl@gmail.com>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20230325074022.9818-4-cnsztl@gmail.com


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