eal/arm: adjust memory barriers for IO on ARMv8

eal/arm: adjust memory barriers for IO on ARMv8

Change the barrier APIs for IO to reflect that Armv8-a is other-multi-copy
atomicity memory model.

Armv8-a memory model has been strengthened to require
other-multi-copy atomicity. This property requires memory accesses
from an observer to become visible to all other observers
simultaneously [3]. This means

a) A write arriving at an endpoint shared between multiple CPUs is
   visible to all CPUs
b) A write that is visible to all CPUs is also visible to all other
   observers in the shareability domain

This allows for using cheaper DMB instructions in the place of DSB
for devices that are visible to all CPUs (i.e. devices that DPDK
caters to).

Please refer to [1], [2] and [3] for more information.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=22ec71615d824f4f11d38d0e55a88d8956b7e45f
[2] https://www.youtube.com/watch?v=i6DayghhA8Q
[3] https://www.cl.cam.ac.uk/~pes20/armv8-mca/

Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Acked-by: Jerin Jacob <jerinj@marvell.com>
Tested-by: Ruifeng Wang <ruifeng.wang@arm.com>