dt-bindings: timer: Add Loongson-1 clocksource

Add devicetree binding document for Loongson-1 clocksource.

Signed-off-by: Keguang Zhang <keguang.zhang@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20230512103724.587760-3-keguang.zhang@gmail.com