Documentation: dt: bindings: Document ARM PSCI enable method

arm,psci is also a valid enable-method for the CPUs on ARM. Document it.
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>

Documentation/devicetree/bindings/arm/cpus.txt

@@ -182,6 +182,7 @@ nodes to be present and contain the properties described below.
 			     "psci"
 			# On ARM 32-bit systems this property is optional and
 			  can be one of:
+			    "arm,psci"
 			    "qcom,gcc-msm8660"
 			    "qcom,kpss-acc-v1"
 			    "qcom,kpss-acc-v2"