clk: loongson2: Add clock definitions for Loongson-2K0300 SoC

The clock controller of Loongson-2K0300 consists of three PLLs, requires
an 120MHz external reference clock to function, and generates clocks in
various frequencies for SoC peripherals.

Clock definitions for previous SoC generations could be reused for most
clock hardwares. There're two gates marked as critical, clk_node_gate
and clk_boot_gate, which supply the CPU cores and the system
configuration bus. Disabling them leads to a SoC hang.

Signed-off-by: Yao Zi <ziyao@disroot.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>