nds32: unexport flush_icache_page

flush_icache_page is only used by mm/memory.c.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Greentime Hu <green.hu@gmail.com>
Cc: Vincent Chen <deanbo422@gmail.com>
Link: http://lkml.kernel.org/r/20200515143646.3857579-3-hch@lst.de


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>