hw/loongarch: Refine system dram memory region

hw/loongarch: Refine system dram memory region

For system dram memory region, it is not necessary to use numa node
information. There is only low memory region and high memory region.

Remove numa node information for ddr memory region here, it can reduce
memory region number on LoongArch virt machine.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Reviewed-by: Song Gao <gaosong@loongson.cn>
Message-Id: <20240515093927.3453674-5-maobibo@loongson.cn>
Signed-off-by: Song Gao <gaosong@loongson.cn>