Switch to INTR_ROOT_COUNT as this better matches the purpose of the
value.

Remove the default from the core.  Better to require the architectures
to declare the type since they will routinely deviate and a default
chosen now will likely be suboptimal.

There is potential for non-root PICs to need per-processor
initialization.  Few root PICs try to propogate the call.  As such add
a pass of calling pic_init_secondary() on all children with a non-root
value.

Differential Revision: https://reviews.freebsd.org/D40474