Per the manual '龙芯 GS264 处理器核用户手册' v1.0, chapter
1.1.5 SEGBITS: the 3A1000 (based on GS464 core) implements
48 virtual address bits in each 64-bit segment, not 40.

Fixes: af868995 ("target/mips: Add Loongson-3 CPU definition")
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Huacai Chen <chenhuacai@loongson.cn>
Message-Id: <20210813110149.1432692-3-f4bug@amsat.org>