ARM: dts: omap3-devkit8000: Fix CONFIG_DM_I2C warning

Seems that u-boot can't probe i2c bus at 2.6Mhz speed, so lower
the speed to the default value 100Khz.

v2: fix i2c1 frequency in the root omap3-u-boot.dtsi include.

Signed-off-by: Anthoine Bourgeois <anthoine.bourgeois@gmail.com>
diff --git a/arch/arm/dts/omap3-u-boot.dtsi b/arch/arm/dts/omap3-u-boot.dtsi
index 32bea6b..96d8ac5 100644
--- a/arch/arm/dts/omap3-u-boot.dtsi
+++ b/arch/arm/dts/omap3-u-boot.dtsi
@@ -78,4 +78,5 @@
 
 &i2c1 {
 	u-boot,dm-spl;
+	clock-frequency = <100000>;
 };