Fix various RK3399 clocks and add support for getting the clock frequency

Fix various RK3399 clocks and add support for getting the clock frequency
of the clocks that we can set.

Assign clock rates (and parents) based on the "assigned-clocks" device tree
property, but only on RK3399 for now as the code for the other Rockchip SoCs
isn't quite ready yet.

Last but not least, fixup a mistake on the firmware for the Theobroma
Systems RK3399-Q7 module such that the "big" cluster uses BPLL as intended.