Add divider and mux debug prints to clocking configuration

[jordens]

CoreClocks provides the resulting frequencies of the clocking configuration. But often that's not sufficient to understand or debug it.
Being able to see the actual divider and clock mux choices would help a lot when trying to match a desired clock configuration with the one the HAL chooses. That can e.g. be addressed with adding a bunch of debug!() prints. Currently the PLL and RCC setup routines are silent and partially black boxes.

@SingularitySurfer

[richardeoin]

+1 from me also. Printing via the log crate expects that users have a working log implementation (for example by copying one from the examples). I think that's reasonable.