bus: Activate INTRNG interrupts in common code

[MFC note: This is not a direct cherry-pick due to API changes in HEAD
which are not present in stable/14.]

We need to call into INTRNG to activate all interrupts on platforms that
use it. Currently, interrupts are only activated in the nexus drivers for
INTRNG platforms, but this does not handle other bus devices such as
gpiobus that manage their own IRQ space.

Reported by: cperciva
Reviewed by: cperciva, jhb
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D47282

(cherry picked from commit c85855a72db9f9d7b4326b676241e1dffabf0fae)