ARM: dts: rockchip: reserve unusable memory region on rk3288

The all current Rockchip SoCs supporting 4GB of ram have problems accessing
the memory region 0xfe000000~0xff000000. This also seems to includes the
rk3368 arm64 soc.

All current code handling dma memory oddities I could find, seem to involve
soc-specific code (zone-dma or so) while this issue is shared between arm32
and arm64 socs from Rockchip, which would need to have this described in
the soc devicetree on both socs.

Limiting the dma-zone alone also does not solve the issue and as the
dma-masks need to be a power-of-two in the kernel, the next lower dma-mask
brings memory usable for dma down to 2GB.

So as a stop-gap block off the affected region to prevent its use by
devices with 4GB of memory, like some recent Chromebooks.

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