MIPS: Loongson-3: Set cache flush handlers to cache_noop

MIPS: Loongson-3: Set cache flush handlers to cache_noop

Loongson-3 maintains cache coherency by hardware, this means:
 1) It's icache is coherent with dcache.
 2) It's dcaches don't alias (maybe depend on PAGE_SIZE).
 3) It maintains cache coherency across cores (and for DMA).

So we can skip most cache flush operations by setting relevant handlers
to `cache_noop' in `r4k_cache_init'.

Signed-off-by: Huacai Chen <chenhc@lemote.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Cc: Steven J . Hill <sjhill@realitydiluted.com>
Cc: Fuxin Zhang <zhangfx@lemote.com>
Cc: Zhangjin Wu <wuzhangjin@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/12752/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>