dt-bindings: clock: Add Loongson-1 clock

Add devicetree binding document and related header file
for the Loongson-1 clock.

Signed-off-by: Keguang Zhang <keguang.zhang@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230321111817.71756-2-keguang.zhang@gmail.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>