arm: plat-orion: Add coherency attribute when setup mbus target

Recent SoC such as Armada 370/XP came with the possibility to deal
with the I/O coherency by hardware. In this case the transaction
attribute of the window must be flagged as "Shared transaction". Once
this flag is set, then the transactions will be forced to be sent
through the coherency block, in other case transaction is driven
directly to DRAM.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Reviewed-by: Yehuda Yitschak <yehuday@marvell.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>