Fix GIC FDT interrupts decoding

Interrupt type in FDT was interpreted incorrectly.
Patch taken from freebsd-arm thread 'GIC - interrupts interpretation in
DTS/FDT':
https://lists.freebsd.org/pipermail/freebsd-arm/2015-August/012145.html

Reviewed by: ian, imp
Obtained from: Semihalf
Sponsored by: Stormshield
Submitted by: Michal Stanek <mst@semihalf.com>
Differential revision: https://reviews.freebsd.org/D4215