int: Set default ARC interrupts to lowest priority to start

The code was setting the interrupt priority to 1 for each
vector to start. But I think it might be more appropriate
to set it to the LOWEST priority. Some ARCs will have
upwards to 15 priorities.

Change-Id: I9330ec8df323fe04ba0751faaafa39694f012364
Signed-off-by: Chuck Jordan <cjordan@synopsys.com>