STM32: document dts porting rules

[erwango]

Due to high number of STM32 SoC references (>900), there are several "rules" or good practices that should be applied when introducing a new SoC into STM32 device tree.
Instantiate a document point to store these rules that people can use as a reference guide and which could be used also for for PR reviews
For instance, some rules where mentioned and discussed in #5883.

This could be stored in a new dts/arm/st/README

[erwango]

Closing this as lack of this document as not been identified as an issue so far