arc: move special-purpose irq priorities to flags

We have already done this on x86 and ARM. The policy is as follows:

* IRQ priority levels starting at 0 all have the same semantics and
do not have special properties. The priority level is either ignored
on arches which do not support programmable priority levels, or lower
priority levels take precedence over higher ones.
* Special-case priorty levels are specified via flags, in which case
the supplied priority level is ignored.

Issue: ZEP-60
Change-Id: Ic603f49299ee1426fb9350ca29d0b8ef96a1d53a
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>