Use mtx_lock_spin in the gic driver

The mutex was changed to a spin lock when the MSI/MSI-X handling was
moved from the gicv2m to the gic driver. Update the calls to lock
and unlock the mutex to the spin variant.

Submitted by: jrtc27 ("Change all the mtx_(un)lock(&sc->mutex) to be the _spin versions.")
Reported by: mw, antranigv@freebsd.am
Sponsored by: The FreeBSD Foundation