MIPS: Loongson64: Add support for the Loongson-2K1000 to get cpu_clock_freq

Get the fixed-clock from the CPU0 node of the device tree.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Qing Zhang <zhangqing@loongson.cn>
Tested-by: Ming Wang <wangming01@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>