dt-bindings: arm: syna: remove unstable remark

dt-bindings: arm: syna: remove unstable remark

Marvell Berlin SoCs (later Syna) bindings were marked as
work-in-progress / unstable in 2015 in commit f07b4e49d27e ("Documentation:
bindings: berlin: consider our dt bindings as unstable").  Almost nine
years is enough, so drop the "unstable" remark and expect usual ABI
rules.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240224084414.6264-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Rob Herring <robh@kernel.org>