dt-bindings: clock: loongson2: Add Loongson-2K0300 compatible

Document the clock controller shipped in Loongson-2K0300 SoC, which
generates various clock signals for SoC peripherals. Differing from
previous generations of SoCs, LS2K0300 requires a 120MHz external clock
input.

Signed-off-by: Yao Zi <ziyao@disroot.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Yanteng Si <siyanteng@cqsoftware.com.cn>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>