Added AD6645 and AD6644 analog to digital converter.

Datasheets:

• AD6644: http://www.analog.com/media/en/technical-documentation/data-sheets/AD6644.pdf
• AD6645: http://www.analog.com/media/en/technical-documentation/data-sheets/AD6645.pdf

Footprint:

• merged with PR #844

Picture:

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