ARM: mvebu: use arm_coherent_dma_ops and re-enable hardware I/O coherency

Now that we have enabled automatic I/O synchronization barriers, we no
longer need any explicit barriers. We can therefore simplify
arch/arm/mach-mvebu/coherency.c by using the existing
arm_coherent_dma_ops instead of our custom mvebu_hwcc_dma_ops, and
re-enable hardware I/O coherency support.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
[Andrew Lunn <andrew@lunn.ch>: Remove forgotten comment]
Signed-off-by: Andrew Lunn <andrew@lunn.ch>