MIPS: Loongson64: Switch to SYNC_R4K

Nowadays SYNC_R4K is performing better than Loongson64's
custom sync mechanism.

Switch to SYNC_R4K to improve performance and reduce code
duplication.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>