mm/unicore32: fix stale comment about VALID_PAGE()

VALID_PAGE() has been removed from kernel long time ago,
so fix the comment.

Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Acked-by: Guan Xuetao <gxt@mprc.pku.edu.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>