Various fixes to the STM32F0 RCC CPU clock startup code

I have an STM32F0DISCOVERY board and some trouble getting the PLLed clocks to work. This set of changes fixes a couple of issues. There were mistakes in the PLL definition table, and the order of the setup seems to be important.

Proper behavior has been checked with a scope on the MCO port using this test code:

https://gist.github.com/onnokort/4663d8ab2b5590864aa6