Add support for STM32WBA's HSE pre-scaler

[Ecco]

This adds support for STM32WBA's prescaler. Several notes:

• RCC_CR should probably be modify-ed than write-en. But that's another PR.
• We may want to add something similar for STM32WB (in l.rs), because on that chip the prescaler option just doesn't do anything.

Also note that the Hse struct only has one field, but I did this to match the structure of other STM32s (as implemented in l.rs).

[Dirbaio]

can you change HSI/HSE so it works like l.rs? i.e an hse field that turns on HSE if Some, an hsi field that turns HSI if true, then mux selects which of them is in use (which must be turned on, of course). https://github.com/embassy-rs/embassy/blob/main/embassy-stm32/src/rcc/l.rs#L41-L42

Please try to keep the RCC API as close as possible as the "newer" RCCs (l.rs, h.rs, f.rs). The WBA RCC is not using the newest API design, it would be preferable if the new features you implement already work like the new design. See this issue for the context on the different API designs we have and why the newer one is better.

[Ecco]

Superseded by #2520