csky/dma: Fixup cache_op failed when cross memory ZONEs

If the paddr and size are cross between NORMAL_ZONE and HIGHMEM_ZONE
memory range, cache_op will panic in do_page_fault with bad_area.

Optimize the code to support the range which cross memory ZONEs.

Changes for V2:
 - Revert back to postcore_initcall

Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Arnd Bergmann <arnd@arndb.de>