Fix MIPS INTRNG (both FDT and non-FDT) behaviour broken by r304459

More changes to MIPS may be required, as commented in D7692, but this
revision aims to restore MIPS INTRNG functionality so we can move on
with working interrupts.

Reported by: yamori813@yahoo.co.jp
Tested by: mizhka (on BCM), sgalabov (on Mediatek)
Reviewed by: adrian, nwhitehorn (older version)
Sponsored by: Smartcom - Bulgaria AD
Differential Revision: https://reviews.freebsd.org/D7692