ARM: dts: rockchip: remove GPU 500 MHz OPP on rk3288

ARM: dts: rockchip: remove GPU 500 MHz OPP on rk3288

The NPLL is the only safe way to generate 500 MHz for the GPU. The
downstream Chrome OS 3.14 kernel ('official' kernel for veyron
devices) re-purposes NPLL to HDMI and hence disables the OPP for
the GPU (see https://crrev.com/c/1574579). Disable it here as well
to keep in sync and avoid problems in case someone decides to
re-purpose NPLL.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
[moved from veyron to general rk3288, as tying up the NPLL for a
 not-that-helpful opp (not really fast but will still generate
 quite a bit of heat) doesn't make so much sense when it will
 keep us from supporting other display modes  in the future]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>