rockchip: rk3288-veyron: Migrate "u-boot, boot0" to "u-boot, spl-boot-order"

"u-boot,spl-boot-order" is more flexible and other rockchip SoCs
has convert to use it, migrate to use the new dts property.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>