MIPS: Loongson64: SMP: Fix up play_dead jump indicator

In play_dead function, the whole 64-bit PC mailbox was used as a indicator
to determine if the master core had written boot jump information.

However, after we introduced CSR mailsend, the hardware will not guarante
an atomic write for the 64-bit PC mailbox. Thus we have to use the lower
32-bit which is written at the last as the jump indicator instead.

Signed-off-by: Lu Zeng <zenglu@loongson.cn>
Signed-off-by: Jun Yi <yijun@loongson.cn>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>