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
Make l1 generic too
Pulls out duplicate calls from f0/rcc.c and uses the common version which
also means that f0 can use rcc_peripheral_enable() now which is in common
but not the old rcc versions.
The STM32F05xxx/06xxx manual describes on p.98 (Sec 7.4.1) the RCC_CR
register, on which it says that bit 24 is the PLLON bit which has to be
enabled before using the PLL. This causes the PLL to be enabled with
rcc_osc_on(PLL).
