ARC: IO: add volatile to accessors

We must use 'volatile' in C-version read/write IO accessors
implementation to avoid merging several reads (writes) into
one read (write), or optimizing them out 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>