ARM: dts: sun8i: set cpu-supply in reference tablet DTSI

All reference design A33 tablets uses DCDC2 of AXP223 as the power
supply of the Cortex-A7 cores.

Set the cpu-supply in the DTSI of sun8i reference tablets.

Signed-off-by: Icenowy Zheng <icenowy@aosc.xyz>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>