Use the new clock API to get the clock frequency from the device tree and the

Use the new clock API to get the clock frequency from the device tree and the
actual hardware that controls the clock.  This gets rid of all the hardcoded
clock frequencies for specific hardware.

Tested by jsg@ on the BBB, mglocker@ on the C.H.I.P. and myself on the
Banana Pi and Raspberry Pi 3.

ok jsg@