Add AVR.td and AVRRegisterInfo.td

Summary:
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.

Reviewers: stoklund

Subscribers: dylanmckay, agnat

Differential Revision: http://reviews.llvm.org/D14684

llvm-svn: 256120