hw/loongarch/virt: Align high memory base address with super page size

hw/loongarch/virt: Align high memory base address with super page size

With LoongArch virt machine, there is low memory space with region
0--0x10000000, and high memory space with started from 0x90000000.
High memory space is aligned with 256M, it will be better if it is
aligned with 1G, which is super page aligned for 4K page size.

Currently linux kernel and uefi bios has no limitation with high
memory base address, it is ok to set high memory base address
with 0x80000000.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Reviewed-by: Song Gao <gaosong@loongson.cn>
Message-Id: <20231127040231.4123715-1-maobibo@loongson.cn>
Signed-off-by: Song Gao <gaosong@loongson.cn>