Add platform architecture to hardware targets in device schema

[glopesdev]

Instead of keeping track of the architecture as a top-level property of the device metadata, it would be preferable to extent the hardware targets attribute. This would allow potentially implementing the same device on multiple target architectures.

Possible structure for the platform architecture attribute has been discussed in #44 (comment) and #57 (comment).

[bruno-f-cruz]

As of SRM 27-03-2025 we agreed to scrape the idea of hardware targets in the device.yml as the file is to be taken as documentation for the abstract interface (that should be sdk-unaware).