743: Add PLL config support for F2 r=Dirbaio a=Gekkio
- minor changes to make the F2 RCC API a bit more flexible
- low-level PLL config with assertions based on datasheet specs. It shouldn't be very difficult to later add a "reverse API" where you pass the clocks you want to a function and it generates a `PLLConfig` struct for you
- PLL API tested on my custom board with 12 MHz HSE as source for PLL to generate max clocks for SYSCLK/AHB/APB/APB1/PLL48
- the example *should* work but is untested since I don't have the Nucleo board 😞
Co-authored-by: Joonas Javanainen <joonas.javanainen@gmail.com>
d600f39260