ARM/dma-mapping: mark various dma-mapping routines static in dma-mapping.c

With the dmabounce removal these aren't used outside of dma-mapping.c,
so mark them static.  Move the dma_map_ops declarations down a bit
to avoid lots of forward declarations.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Marc Zyngier <maz@kernel.org>