LoongArch: Update dts for Loongson-2K2000 to support GMAC/GNET

Current dts file for Loongson-2K2000's GMAC/GNET is incomplete, both irq
and phy descriptions are missing. Add them to make GMAC/GNET work.

Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>