ARC: Cache: Disable IOC by default

We'd like to keep IOC HW at the same state as t is right after reset when we
start Linux kernel so there will be no re-configuration of IOC on the go.

The point is U-Boot doesn't benefit a lot from IOC as it doesn't do a
lot of DMA operations especially on multiple cores simultaneously.

At the same time re-configuration of IOC in run-time might become quite
a tricky experience because we need to make sure there're no DMA
trannsactions in flight otherwise unexpected consequencses might affect
us much later and debugging those kinds of issues will be a real
nightmare.

That said let's make our life easier a little bit.

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