Prepare the Renode models for changing the PeripheralRegister underlying type

As the `PeripheralRegister` underlying value type is about to change from `ulong` to `uint` all models have to be adjusted accordingly.
This change is compatible with both the upcoming change and with the current master/nightly.

Change-Id: I7422ae92a2767288cbc899602a8a3b7a45080f3c