Hello. These two are neither supported in v5.15. Moreover, I do not remember where I read it but with v6.x, mbed team has decided to remove the support for a considerable number of previously supported boards. This is for keeping the size of the whole frame work smaller and reducing the workload of the team. Here, you can find an example of adding a new STM32 target. Once you understand how the targets folder is organized, it is pretty straight forward adding a new target mcu. There is also a readme in TARGET_STM. It does not explain every single step but it has a lot of hints about adding a new target mcu. H753ZI should be easy but H755ZI requires some more tricks since it is a dual core mcu.
One small note. You might have to modify index.json in mbed-os/tools/arm_pack_manager.