MIPS: Loongson64: Add Loongson-2K1000 early_printk_port

Distinguish between Loongson-3A series CPU and Loongson-2K1000 CPU UART0.

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>