Move `mbedtls_dev` to framework submodule

[davidhorstmann-arm]

Move the mbedtls_dev python module into the mbedtls-framework repository. This PR pairs with Mbed-TLS/mbedtls-framework#15.

PR checklist

Please tick as appropriate and edit the reasons (e.g.: "backport: not needed because this is a new feature")

• [x] changelog not required - non-user-facing change
• [x] 3.6 backport #9131
• [x] 2.28 backport not required - no framework submodule for 2.28
• [x] tests not required - internal script motion only - current tests should suffice

[davidhorstmann-arm]

Otherwise, have you checked that you can play the same move game subsequently on another set of mbedtls files, tests/data_files/* for example without merge trouble in framework?

I have now added to the script so that it can do this. The result is in Mbed-TLS/mbedtls-framework#18, which is based on the corresponding PR in the framework submodule.

[ronald-cron-arm]

@davidhorstmann-arm @bensze01 I've just updated the framework to the merge of mbedtls-framework:#15.