bus: Activate INTRNG interrupts in common code

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