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 <manu@bidouilliste.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>