arm: Handle GIC_IVAR_VGIC in the gic driver

We don't have a GICv2 vgic so can just return 0.

Sponsored by:	Arm Ltd
Differential Revision:	https://reviews.freebsd.org/D53662

sys/arm/arm/gic.c

@@ -514,6 +514,9 @@ arm_gic_read_ivar(device_t dev, device_t child, int which, uintptr_t *result)
 		    ("arm_gic_read_ivar: Invalid bus type %u", sc->gic_bus));
 		*result = sc->gic_bus;
 		return (0);
+	case GIC_IVAR_VGIC:
+		*result = 0;
+		return (0);
 	}
 
 	return (ENOENT);