't have scope to measure clock rate, but you still can give it a try.  
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please follow up https://tinkerboarding.co.uk/wiki/index....are#Kernel to download kernel of Tinkerboard.
and push my attachment(you can modify frequency as you want) to kernel/arch/arm/boot/dts/overlays/

then, you can start to build kernel. after building, copy kernel/arch/arm/boot/dts/overlays/i2c-clock-freq.dtbo to /boot/overlays/ of tinkerboard.
and modify /boot/hw_intf.conf as below

intf:i2c1=on
intf:i2c4=on
intf:dtoverlay=i2c-clock-freq

after rebooting, check with "xxd /proc/device-tree/i2c@ff650000/clock-frequency"   

P.S. mapping table as below:
i2c@ff650000 ->I2C0
i2c@ff140000 ->I2C1
i2c@ff660000 ->I2C2
i2c@ff150000 ->I2C3
i2c@ff160000 ->I2C4
i2c@ff170000 ->I2C5