MIPS: Loongson64: smp: Use nudge_writes instead of wbflush

wbflush here intends to let other cores see the results ASAP,
nudge_writes fits this purpose better.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>