Rename rcc_ppre1_frequency and rcc_ppre2_frequency

Rename rcc_ppre1_frequency and rcc_ppre2_frequency to rcc_apb1_frequency and rcc_apb2_frequency
Also add rcc_ahb_frequency (although it is not set correctly in all cases) which will be fixed by
the rcc commits later. Also fixup the only use in the library of these variables, the USART code.

And fix the typos that resulted