As those hangs/lockups are related to so many different changes it's
really hard to debug them.
This bug seems to be specific to the slow arch clock that affects
stability only when kernel locking code and symbols layout trigger some
very specific timing.
Enabling CONFIG_PROVE_LOCKING seems to make issue go away but it affects
so much code it's hard to tell why it actually matters.
Same for disabling CONFIG_SMP. I noticed Broadcom's SDK keeps it
disabled. I tried it and it improves stability (I had 3 devices with 6
days of uptime and counting) indeed. Again it affects a lot of kernel
parts so it's hard to tell why it helps.
Unless someone comes up with some magic solution I'll probably try
building BCM53573 images without CONFIG_SMP for my personal needs.