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

I downloaded PyCortexMDebug today to try it out with my system, and I got some crashes out of the box trying to run the `svd` command.  This patch contains the fixes I needed to make:

1. apparently sometimes p.description wasn't a string.  Adding a `str()` call in pysvd.py makes it work.

2. try getting the `name` and `description` fields even for derived peripherals

Hopefully these changes won't break anything - let me know if you have any other changes/suggestions you'd like to see before pulling.