ARM/dma-mapping: use dma-direct unconditionally

Use dma-direct unconditionally on arm.  It has already been used for
some time for LPAE and nommu configurations.

This mostly changes the streaming mapping implementation and the (simple)
coherent allocator for device that are DMA coherent.  The existing
complex allocator for uncached mappings for non-coherent devices is still
used as is using the arch_dma_alloc/arch_dma_free hooks.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Andre Przywara <andre.przywara@arm.com> [highbank]
Tested-by: Marc Zyngier <maz@kernel.org>