Fix crashes in 'svd' with Toshiba SVD xml file.

- apparently sometimes p.description wasn't a string.  Adding a str()
  in pysvd.py makes it work.
- try getting the 'name' and 'description' fields even for derived
  peripherals
Merge pull request #1 from carlnorum/toshiba

Fix crashes in 'svd' with Toshiba SVD xml file.