arc: initialize the i-cache ONLY if it is present

The aux register named _ARC_V2_I_CACHE_BUILD can be read
to determine if the ARC has an i-cache. If it does not,
don't attempt to initialize it because this would cause an exception.

Change-Id: I3ff519befcc5ebb7745b58401f12cf3015a9e2e9
Signed-off-by: Chuck Jordan <cjordan@synopsys.com>