ARM: dts: Switch i2c0 to 400kHz on rk3288-evb-rk808

We should be able to talk to the PMIC at 400kHz.  No need to talk at
the slow 100kHz.

As measured by ftrace (with a bunch of extra patches, since cpufreq
for rk808 hasn't landed yet):
  before this change: cpu0_set_target() => ~500us
  after this change:  cpu0_set_target() => ~300us
Signed-off-by: Doug Anderson <dianders@chromium.org>
Reviewed-by Addy Ke <addy.ke@rock-chips.com>
Tested-by Addy Ke <addy.ke@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

arch/arm/boot/dts/rk3288-evb-rk808.dts

@@ -18,6 +18,7 @@ / {
 };
 
 &i2c0 {
+	clock-frequency = <400000>;
 	status = "okay";
 
 	rk808: pmic@1b {