Fix the GIC ACPI cross reference value.

To support INTRNG with ACPI we need to set a non-zero cross reference value
for the interrupt controller. The GICv3 driver already had this value set,
however it was missed in the GICv2 driver. Fix this by setting xref to the
correct value.

Approved by:	re (gjb)

sys/arm/arm/gic_acpi.c

@@ -34,6 +34,7 @@
  * SUCH DAMAGE.
  */
 
+#include "opt_acpi.h"
 #include "opt_platform.h"
 
 #include <sys/cdefs.h>
@@ -217,7 +218,7 @@ gic_acpi_attach(device_t dev)
 	if (err != 0)
 		return (err);
 
-	xref = 0;
+	xref = ACPI_INTR_XREF;
 
 	/*
 	 * Now, when everything is initialized, it's right time to