540: Initial support for STM32F3 r=Dirbaio a=VasanthakumarV
The [companion PR](https://github.com/embassy-rs/stm32-data/pull/109) in `stm32-data` should be merged before this PR.
The examples were tested on an STM32F303VC MCU.
Co-authored-by: VasanthakumarV <vasanth260m12@gmail.com>
Co-authored-by: Dario Nieuwenhuis <dirbaio@dirbaio.net>
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