The relationship between clockspeed and power consumption is nonlinear.

http://electronics.stackexchange.com/questions/122050/what-l...

(see graph in the first answer)

Also, it's not known that the TPU have a way to allow to increase the clockspeed arbitrarily, nor is it known whether their architecture is capable of ensuring correctness at arbitrary clock frequencies. Some architectures make assumptions like "The time for this gate to reach saturation is very small compared to the clock frequency, so we'll pretend that it's instantaneous."