ARM: sun6i: Define the A31 CPUs enable-method

That will allow to use the CPU_METHOD_OF_DECLARE definition we did previously.
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>

arch/arm/boot/dts/sun6i-a31.dtsi

@@ -27,6 +27,7 @@ aliases {
 
 
 	cpus {
+		enable-method = "allwinner,sun6i-a31";
 		#address-cells = <1>;
 		#size-cells = <0>;