I have started a port of RIOT to the stm32h7 family. I searched the forum and did not see any mention of anyone else working on this.
Specifically I am using a Nucleo-H723ZG as my test bed. At the moment, I can flash it and run the boot sequence up to the point of configuring the PLL.
Here is a very rough (very full of hacks as of this post) and ready preview of the work: