arm64: dts: rockchip: add ap_pwroff and ddrio_pwroff pins for rk3399

There are two sleep related pins on rk3399: ap_pwroff and ddrio_pwroff.
Let's add the definition of these two pins to rk3399's main dtsi file so
that boards can use them.

These two pins are similar to the global_pwroff and ddrio_pwroff pins in
rk3288 and are expected to be used in the same way: boards will likely
want to configure these pinctrl settings in their global pinctrl hog
list.

Note that on rk3288 there were two additional pins in the "sleep"
section: "ddr0_retention" and "ddr1_retention".  On rk3288 designs these
pins appeared to actually route from rk3288 back to rk3288.  Presumably
on rk3399 this is simply not needed since the pins don't appear to exist
there.

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