Rename rcc_ppre1_frequency and rcc_ppre2_frequency

The original names of rcc_ppre1_frequency and rcc_ppre2_frequency were confusing to me. This updates these variables (that are used in computing USART baudrates and potentially timer periods) to have names reflecting their basis. In particular apb1 and abp2 clocks. I've also added a new counter rcc_ahb_frequency which is generally going to be the SYSCLK frequency. 
Seems to me that `rcc_ahb_frequency = 48000000;` is missing here...