Support STM32H7R3/7S3

[richardeoin]

ST have added a new sub-family called STM32H7R3/7S3. The reference manual is RM0477.

These parts are not fully supported by stm32h7xx-hal currently. This issue tracks the status of using Rust on these parts, either using stm32h7xx-hal or otherwise.

• Identify the scope of the differences between STM32H7R3/7S3 and the existing STM32H7 sub-families
• Decide if stm32h7xx-hal is the right place to support these parts. If the overhead of supporting the differences is low, then the answer should be yes
• Add support

PRs for adding support are welcome, but please comment to address the first two items first.

Differences between STM32H7R3/7S3 and the existing STM32H7 sub-families

• New peripherals: UCPD PKA MCI ...
• Additional PLL outputs S and T

[Dirbaio]

H7RS is more like a whole new family. it's more similar to H5 or U5 than to the old H7's. For example DMA is completely new (GPDMA+HPDMA), instead of DMA+BDMA+DMAMUX.

It feels like "marketing" that they decided to name them "H7"... ST themselves are maintaining support for it separately: see https://github.com/STMicroelectronics/STM32CubeH7 vs https://github.com/STMicroelectronics/STM32CubeH7RS

(dropping it here in case people are interested in Rust on STM32H7RS: the embassy-stm32 HAL does have support for H7RS now)