ARM: dts: sunxi: Add cpu-supply for Olimex A20 EVB

sun7i-a20-olimex-som-evb.dts doesn't contain cpu-supply needed for
voltage-scaling with cpufreq-dt so define it.
The default voltages are defined in sun7i-a20.dtsi.

Signed-off-by: Emmanuel Vadot <[e-mail]>
Signed-off-by: Maxime Ripard <[e-mail]>

arch/arm/boot/dts/sun7i-a20-olimex-som-evb.dts

···
 	status = "okay";
 };
 
+&cpu0 {
+	cpu-supply = <&reg_dcdc2>;
+};
+
 &codec {
 	status = "okay";
 };