clk: clk-loongson2: add clock controller driver support

This driver provides support for clock controller on Loongson-2 SoC,
the Loongson-2 SoC uses a 100MHz clock as the PLL reference clock,
there are five independent PLLs inside, each of which PLL can
provide up to three sets of frequency dependent clock outputs.

Signed-off-by: Yinbo Zhu <zhuyinbo@loongson.cn>
Link: https://lore.kernel.org/r/20230323025229.2971-2-zhuyinbo@loongson.cn
Signed-off-by: Stephen Boyd <sboyd@kernel.org>