Fix bugs in interrupts allocation on ARM64

[zbb]

Summary

1. Separate interrupt descriptors lookup from allocation. It was possible to perform config on non-existing interrupt simply by allocating spurious descriptor.
2. Must lock the interrupt descriptors table lookup to avoid mismatches. This ought to prevent trouble while setting up new interrupt and dispatching existing one.
3. Use spin mutex rather than sleep mutex. This is mainly due to lock in arm_dispatch_intr

This should be eventually changed to a lock-less solution without
walking through a linked list on each interrupt.

Reviewed by:
Obtained from: Semihalf
Sponsored by: Cavium
Differential Revision:

[wma]

As we discussed today, looks fine to me.

[zbb]

Don't lock PIC_ methods.
Not only this implies some behavior on methods but also is not necessary because we only need lock on the interrupt descriptors list and interrupt resources are locked by the resource manager.