ARC: IO: add compiler barriers to IO accessors

We must use compiler barriers in C-version read/write IO accessors
before and after operation (read or write) so it won't be reordered
by compiler.

Fixes commit 07906b3d ("ARC: Switch to generic accessors")

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