Fall back to caption from module if instance caption is missing

[Rahix]

From Rahix/avr-device#26

@jwagen: There is also an issue that module list contain the caption attribute of that module, but the instances of that module does not contain the caption attribute. Currently atdf2svd expects it to be on the instance.

@Rahix: Would adding a fall-back for the other tag fix this, or is a more elaborate distinction needed?

@jwagen: Yes a fall-back does seem to be sufficient. So I don't think something more elaborate is necessary at least for now.

[Rahix]

Here,

atdf2svd/src/atdf/peripheral.rs

Line 56 in 52fd47b

description: instance.attr("caption").ok().cloned(),

only the caption from the <instance ...> is used. That's where a fallback to the <module ...> which is parsed further up should be added.