intrng: fix INTR_ROOT_* constants

Switch to INTR_ROOT_COUNT as this name better describes its purpose.

Remove the default INTR_ROOT_IRQ from the core. Define it (redundantly)
in each architecture's header, but now placed alongside its sibling
values (if defined by the platform, e.g. arm64 INTR_ROOT_FIQ).

Reviewed by: mhorne
Pull Request: https://github.com/freebsd/freebsd-src/pull/1280