ARM: socfpga: dts: add enable-method property for cpu nodes

Add the enable-method property for the cpu node on socfpga.dtsi and
socfpga_arria10.dtsi. This is for CPU_METHOD_OF_DECLARE to use to enable
the secondary core.
Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>

arch/arm/boot/dts/socfpga.dtsi

@@ -36,6 +36,7 @@ aliases {
 	cpus {
 		#address-cells = <1>;
 		#size-cells = <0>;
+		enable-method = "altr,socfpga-smp";
 
 		cpu@0 {
 			compatible = "arm,cortex-a9";

arch/arm/boot/dts/socfpga_arria10.dtsi

@@ -36,6 +36,7 @@ aliases {
 	cpus {
 		#address-cells = <1>;
 		#size-cells = <0>;
+		enable-method = "altr,socfpga-a10-smp";
 
 		cpu@0 {
 			compatible = "arm,cortex-a9";