kbuild: Enable dtc graph_port warning by default

All the 'graph_port' warnings have been fixed or have pending fixes, so
we can enable it by default now.
Signed-off-by: Rob Herring <robh@kernel.org>

scripts/Makefile.lib

@@ -251,7 +251,6 @@ DTC_FLAGS += -Wno-unit_address_vs_reg \
 	-Wno-avoid_unnecessary_addr_size \
 	-Wno-alias_paths \
 	-Wno-graph_child_address \
-	-Wno-graph_port \
 	-Wno-simple_bus_reg \
 	-Wno-unique_unit_address \
 	-Wno-pci_device_reg