dt-bindings: riscv: spacemit: Add OrangePi R2S board

dt-bindings: riscv: spacemit: Add OrangePi R2S board

Document the compatible string for the OrangePi R2S board [1], which
is marketed as using the Ky X1 SoC but is in fact identical in die
and package to the SpacemiT K1 SoC [2].

Link: http://www.orangepi.org/html/hardWare/computerAndMicrocontrollers/details/Orange-Pi-R2S.html
Link: https://www.spacemit.com/en/key-stone-k1
Signed-off-by: Michael Opdenacker <michael.opdenacker@rootcommit.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Yixun Lan <dlan@gentoo.org>
Link: https://lore.kernel.org/r/20251112044426.2351999-2-michael.opdenacker@rootcommit.com
Signed-off-by: Yixun Lan <dlan@gentoo.org>