Add AVR.td and AVRRegisterInfo.td
This adds the core AVR TableGen file, along with the register descriptions.
Lines in AVR.td which require other TableGen files which haven't been committed
yet are commented out.
This is a fairly trivial patch, and should only require a quick review.
I kept the line width smaller than 80 columns, but there are a few exceptions
because I'm not sure how to split a string over several lines.
Subscribers: dylanmckay, agnat
Differential Revision: http://reviews.llvm.org/D14684