ARM: dts: rockchip: add cpu-supply property for cpu node of rk3229-evb

This patch adds the cpu-supply property so that cpu can do dvfs
on rk3229-evb board.

Signed-off-by: Finley Xiao <finley.xiao@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>