MIPS: CPU_LOONGSON2EF need software to maintain cache consistency

CPU_LOONGSON2EF need software to maintain cache consistency,
so modify the 'cpu_needs_post_dma_flush' function to return true
when the cpu type is CPU_LOONGSON2EF.

Cc: stable@vger.kernel.org
Signed-off-by: Lichao Liu <liulichao@loongson.cn>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>