arm: mvebu: Add hardware I/O Coherency support

Armada 370 and XP come with an unit called coherency fabric. This unit
allows to use the Armada 370/XP as a nearly coherent architecture. The
coherency mechanism uses snoop filters to ensure the coherency between
caches, DRAM and devices. This mechanism needs a synchronization
barrier which guarantees that all the memory writes initiated by the
devices have reached their target and do not reside in intermediate
write buffers. That's why the architecture is not totally coherent and
we need to provide our own functions for some DMA operations.

Beside the use of the coherency fabric, the device units will have to
set the attribute flag of the decoding address window to select the
accurate coherency process for the memory transaction. This is done
each device driver programs the DRAM address windows. The value of the
attribute set by the driver is retrieved through the
orion_addr_map_cfg struct filled during the early initialization of
the platform.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Reviewed-by: Yehuda Yitschak <yehuday@marvell.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>