arm64: dts: rockchip: Fix override mode for rk3399-kevin panel

When I re-posted Sean's original commit to add the override mode for
the kevin panel, for some reason I didn't notice that the pixel clock
wasn't quite right.  Looking at /sys/kernel/debug/clk/clk_summary on
downstream kernels it can be seen that the VOP clock is supposed to be
266,666,667 Hz achieved by dividing the 800 MHz PLL by 3.

Looking at history, it seems that even Sean's first patch [1] had this
funny clock rate.  I'm not sure where it came from since the commit
message specifically mentioned 26666 kHz and the Chrome OS tree [2]
can be seen to request 266667 kHz.

In any case, let's fix it up.  This together with my patch [3] to do
the proper rounding when setting the clock rate makes the VOP clock
more proper as seen in /sys/kernel/debug/clk/clk_summary.

[1] https://lore.kernel.org/r/20180206165626.37692-4-seanpaul@chromium.org
[2] https://chromium.googlesource.com/chromiumos/third_party/kernel/+/chromeos-4.4/drivers/gpu/drm/panel/panel-simple.c#1172
[3] https://lkml.kernel.org/r/20191003114726.v2.1.Ib233b3e706cf6317858384264d5b0ed35657456e@changeid

Fixes: 84ebd2da6d ("arm64: dts: rockchip: Specify override mode for kevin panel")
Cc: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20191008124949.1.I674acd441997dd0690c86c9003743aacda1cf5dd@changeid
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

arch/arm64/boot/dts/rockchip/rk3399-gru-kevin.dts

@@ -44,7 +44,7 @@
 		power-supply = <&pp3300_disp>;
 
 		panel-timing {
-			clock-frequency = <266604720>;
+			clock-frequency = <266666667>;
 			hactive = <2400>;
 			hfront-porch = <48>;
 			hback-porch = <84>;