MIPS: Loongson64: DTS: Add RTC support to Loongson-2K1000

The module is now supported, enable it.

Acked-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Signed-off-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>