dt-bindings: clock: add loongson-2 boot clock index

The Loongson-2 boot clock was used to spi and lio peripheral and
this patch was to add boot clock index number.

Signed-off-by: Yinbo Zhu <zhuyinbo@loongson.cn>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230323025229.2971-1-zhuyinbo@loongson.cn
Signed-off-by: Stephen Boyd <sboyd@kernel.org>