Expose peripheral registers when enabling the unstable-pac feature. about embassy

[mattico]

I was just going to open an issue about this. I have a similar use-case, where I have an ADC which has a dual-SPI-like interface connected to an STM32 SPI in Receive-only-slave mode, plus some unusual DMA configuration.

Previously I was using stm32-rs/stm32h7xx-hal which has fairly comprehensive support for peripheral configuration, but still maybe 10% of my peripherals had some configuration which wasn't supported natively in the HAL. When this situation came up I had a few choices:

1. Use the HAL peripheral, but after the HAL configures it change a few values in the registers directly.
2. Copy the HAL peripheral implementation into my project and modify it to work as necessary.
3. Add support for the feature I needed to upstream, usually after having tested the changes by doing option 1 or 2.

Adding upstream support was preferred but was often way more work, as I'd need to come up with an expansive new general interface for the feature and implement it for all of the supported microcontrollers - not just a specific hack for my specific microcontroller. Often it's not practical for an upstream HAL to support the full breadth of a peripheral's possible configurations, as the implementation and interface would be massive and complex.

Options 1 and 2 would often break when updating the HAL since they relied so much on HAL internals, but they were still very useful and convenient compared to having to create a whole new peripheral from scratch based on just the PAC. embassy-stm32 sealing away its internals makes life more difficult for me.