Only decode fdt data which belongs to the GIC controller.

The interrupts-extended property is a list of controller-specific
interrupt tuples for more than one controller. The decode routine of
every PIC gets called in the pre-INTRNG code (nexus doesn't know which
device instance belongs to which fdt node), so the GIC code has to
check each FDT node it is asked to decode to ensure it is the owner.

Because in the pre-INTRNG world there can only be one instance of a GIC,
it's safe to cache the results of a positive lookup in a static variable
to avoid the expensive lookups on subsequent calls.

Submitted by: Svatopluk Kraus <onwahe@gmail.com>
Differential Revision: https://reviews.freebsd.org/D2345