Documentation: dt: bindings: Document Allwinner A31 enable method

Document the necently introduced A31 enable-method as a valid option.
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>

Documentation/devicetree/bindings/arm/cpus.txt

@@ -183,6 +183,7 @@ nodes to be present and contain the properties described below.
 			# On ARM 32-bit systems this property is optional and
 			  can be one of:
 			    "arm,psci"
+			    "allwinner,sun6i-a31"
 			    "qcom,gcc-msm8660"
 			    "qcom,kpss-acc-v1"
 			    "qcom,kpss-acc-v2"