hw/loongarch/virt: Add high MMIO support with GPEX host

With high MMIO supported, its base address comes from high end of
physical address space. Also add high MMIO support with GPEX host bridge.

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