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).
