ARM: dts: rockchip: Add DDR retention/poweroff to rk3288-veyron hogs

Even though upstream Linux doesn't yet go into deep enough suspend to
get DDR into self refresh, there is no harm in setting these pins up.
They'll only actually do something if we go into a deeper suspend but
leaving them configed always is fine.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>