hw/loongarch: Improve acpi dsdt table

Cleanup the previous pci information in acpi dsdt table.
And using the common acpi_dsdt_add_gpex function to build
the gpex and pci information.

Signed-off-by: Xiaojuan Yang <yangxiaojuan@loongson.cn>
Acked-by: Song Gao <gaosong@loongson.cn>
Message-Id: <20220908094623.73051-10-yangxiaojuan@loongson.cn>
Signed-off-by: Song Gao <gaosong@loongson.cn>