ARC: IO: add MB for __raw_* memory accessors

We add memory barriers for __raw_readX / __raw_writeX accessors same
way as it is done for readX and writeX accessors as lots of U-boot
driver uses __raw_readX / __raw_writeX instead of proper accessor
with barrier.

It will save us from lot's of debugging in the future and it is OK
as U-Boot is not that performance oriented as real run-time
software like OS or user bare-metal app so we may afford being not
super fast as we only being executed once.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>