482: Add MCO peripheral. r=Dirbaio a=matoushybl
This PR adds an abstraction over STM32 RCC feature called MCO (Microcontroller Clock Output). The clock output can bind to several clock sources and then can be scaled using a prescaler.
Given that from the embassy ecosystem the RCC is generaly invisible to the user, the MCO was implemented as a separate peripheral bound to the pin where the clock should appear.
Co-authored-by: Matous Hybl <hyblmatous@gmail.com>
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