Add support for STM32N6 series chips

[SomeSpaceNerd]

Add support for the newer STM32N6 chips and their features, or at least the (seemingly more widely available) N657 chips as used in the NUCLEO-N657X0-Q.

[jmole]

Is anyone working on this right now? I'm interested to hear your plans for dealing with Secure Boot, which is enabled by default on the STM32N6 series.

[fpistm]

Hi @jmole
STM32N6 is more complex than other series.
Due to external memory requirement, security and usage.
Currently, I've not planned to work on it. Anyway any contribution are welcome like it was done for STM32MP1.